Method for indicating channel access type in wireless communication system, and apparatus therefor

ABSTRACT

Disclosed is a method for a station (STA) for accessing a channel in a wireless communication system. The method for a STA for accessing a channel in a wireless communication system comprises the steps of: receiving a beacon frame comprising a traffic indicator map (TIM); and transmitting a power save (PS)-Poll frame if buffered traffic is indicated by TIM to the STA, wherein the PS-Poll frame is transmitted during a PS-Poll dedicated restricted access window (RAW) by and/or during an additional RAW, the PS-Poll dedicated RAW being allocated to transmit the PS-Poll frame, and the additional RAW being additionally allocated subsequent to the PS-Poll-dedicated RAW.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method of indicating a channel access type in awireless LAN system and apparatus for supporting the same.

BACKGROUND ART

Recently, various kinds of wireless communication technologies have beendeveloped together with the developments of the informationcommunication technology. Particularly, wireless LAN (WLAN) is thetechnology for accessing Internet by wireless in a home, a company or aspecific service provided area using such a mobile user equipment as apersonal digital assistant (PDA), a laptop computer, a portablemultimedia player (PMP) and the like based on a radio frequencytechnology.

In order to overcome the limitation put on a communication speed pointedout as a weak point of WLAN, the recent technology standard hasintroduced a system having an enhanced speed and reliability of anetwork and an extended operating distance of a wireless network. Forinstance, IEEE 802.11n has introduced the application of MIMO (MultipleInputs and Multiple Outputs) that uses multiple antennas at both endsincluding a transmitting unit and a receiving unit in order to supporthigh throughput for a data processing speed over maximum 540 Mbps,minimize transmission error, and optimize a data rate or speed.

DISCLOSURE OF THE INVENTION Technical Tasks

One technical task of the present invention is to provide an enhancedmethod of accessing a channel in a wireless communication system, andpreferably, in a wireless LAN (WLAN) system and apparatus therefor.

Another technical task of the present invention is to provide a methodof preventing an unnecessary power consumption and transmission delay ofa user equipment due to a contention based channel access operation in aWLAN system and apparatus therefor.

A further technical task of the present invention is to provide anopportunity for attempting a channel access additionally if a stationfails in performing a channel access successfully.

Technical tasks obtainable from the present invention are non-limited bythe above-mentioned technical tasks. And, other unmentioned technicaltasks can be clearly understood from the following description by thosehaving ordinary skill in the technical field to which the presentinvention pertains.

Technical Solutions

In a 1^(st) technical aspect of the present invention, provided hereinis a method of performing a channel access, which is performed by astation (STA) in a wireless communication system, the method includingthe steps of receiving a beacon frame containing a TIM (trafficindication map) and if a presence of a traffic buffered for the STA isindicated by the TIM, transmitting a PS-Poll (Power Save-Poll) frame,wherein the PS-Poll frame is transmitted during at least one of aPS-Poll dedicated RAW (restricted access window) assigned to transmitthe PS-Poll frame and an additional RAW additionally assigned behind thePS-Poll dedicated RAW.

The STA may attempt a transmission of the PS-Poll frame in a PS-Pollinterval assigned for the STA within the PS-Poll dedicated RAW. In doingso, the PS-Poll interval may be configured different per STA receivingthe indication of the presence of the buffered traffic through the TIMwithin the PS-Poll dedicated RAW.

If the PS-Poll is not successfully transmitted during the PS-Polldedicated RAW, the STA may transmit the PS-Poll frame during theadditional RAW.

In doing so, if the STA fails to transmit the PS-Poll frame in thePS-Poll interval or is unable to receive an ACK (acknowledgement) framein response to the PS-Poll despite transmitting the PS-Poll frame in thePS-Poll interval, the STA may determine that the PS-Poll frame was notsuccessfully transmitted.

On the other hand, if the presence of the traffic buffered for the STAis not indicated by the TIM or the STA successfully transmits thePS-Poll frame during the PS-Poll dedicated RAW, the STA may receive aUTA (UL transmission allowance) frame during the additional RAW and maythen attempt the channel access during the additional RAW.

At least one of EDCA parameters applied to transmitting the PS-Pollframe during at least one of the PS-Poll dedicated RAW and theadditional RAW may be equivalent to or smaller than an EDCA parameterapplied to transmitting an audio traffic.

In this case, the EDCA parameter may include at least one of CWmin(minimum Contention Window), CWmax (maximum Contention Window) and AIFSN(Arbitration Inter-Frame Spacing Number).

The STA may further receive a data frame during the additional RAW. Inthis case, the STA may receive a UTA (UL transmission allowance frame)during the additional RAW after receiving the data frame.

For another instance, a transmission priority of the PS-Poll frame maybe set equivalent to that of an audio traffic during at least one of thePS-Poll dedicated RAW and the additional RAW.

In a 2^(nd) technical aspect of the present invention, provided hereinis a method of performing a channel access, which is supported by an AP(access point) in a wireless communication system, including the stepsof transmitting a beacon frame containing a TIM (traffic indication map)and receiving a PS-Poll (Power Save-Poll) frame from a paged STAreceiving an indication of a presence of a buffered traffic the TIM,wherein the PS-Poll frame is received during at least one of a PS-Polldedicated RAW (restricted access window) assigned to receive the PS-Pollframe and an additional RAW additionally assigned behind the PS-Polldedicated RAW.

The PS-Poll dedicated RAW may be a sum of a PS-Poll interval fortransmitting the PS-Poll frame per the paged STA.

The AP may receive the PS-Poll during the additional RAW from at leastone STA failing in transmitting the PS-Poll frame successfully in itsown PS-Poll interval among the pated STAs.

The AP may transmit a UTA (UL transmission allowance) frame indicatingthat the channel access of the STA is allowed during the additional RAW.In doing so, the UTA frame may be transmitted while a channel is idleover a prescribed time during the additional RAW.

The UTA frame may be transmitted to the paged STA by unicast ormulticast. For another instance, the UTA frame may be transmitted bybroadcast.

A transmission priority of the PS-Poll frame may be set higher than thatof an audio traffic during at least one of the PS-Poll dedicated RAW andthe additional RAW. For another instance, a transmission priority of thePS-Poll frame may be set equivalent to that of an audio traffic duringat least one of the PS-Poll dedicated RAW and the additional RAW.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention.

FIG. 1 is a diagram for one example of a structure of IEEE 802.11 systemto which the present invention is applicable.

FIG. 2 is a diagram for another example of a structure of IEEE 802.11system to which the present invention is applicable.

FIG. 3 is a diagram for a further example of a structure of IEEE 802.11system to which the present invention is applicable.

FIG. 4 is a diagram for one example of a structure of WLAN system.

FIG. 5 is a diagram for one example of the structure of a data linklayer and a physical layer on IEEE 802.11 system to which the presentinvention is applicable.

FIG. 6 is a diagram to describe a general link setup process in a WLANsystem to which the present invention is applicable.

FIG. 7 shows one example of a MAC frame format of IEEE 802.11 system towhich the present invention is applicable.

FIG. 8 shows one example of HT format of HT Control field in MAC frameaccording to FIG. 7.

FIG. 9 shows one example of VHT format of HT Control field in MAC frameaccording to FIG. 7.

FIG. 10 shows one example of PPDU frame format of IEEE 802.11n system towhich the present invention is applicable.

FIG. 11 shows one example of VHT PPDU frame format of IEEE 802.11acsystem to which the present invention is applicable.

FIG. 12 is a diagram to describe a back-off process in a WLAN system towhich the present invention is applicable.

FIG. 13 is a diagram to describe a hidden node and an exposed node.

FIG. 14 is a diagram to describe RTS and CTS.

FIG. 15 is a diagram for one example of relation of IFS.

FIG. 16 is a diagram to describe a power management operation.

FIGS. 17 to 19 are diagrams to describe operations of an STA havingreceived TIM in detail.

FIG. 20 is a diagram for one example of TIM element format.

FIG. 21 shows one example of U-APSD coexistence element format.

FIG. 22 is a diagram to describe operations of STA according to PS-Pollmechanism and U-APSD mechanism.

FIG. 23 is a diagram for one example of a case that PS-Poll framecollides in a hidden node environment.

FIG. 24 is a diagram for one example of PS-Poll contention mechanism ina hidden node environment.

FIG. 25 is a diagram for one example of NDP PS-Poll frame.

FIG. 26 is a diagram for one example of a channel access operation ofSTA using an extended slot time.

FIG. 27 is another diagram for one example of a channel access operationof STA using an extended slot time.

FIG. 28 is a diagram for one example of PS-Poll interval configured perSTA according to one embodiment of the present invention.

FIGS. 29 to 34 are diagrams to describe a channel access operation ofSTA according to one embodiment of the present invention.

FIG. 35 is a diagram for one example of NDP ACK frame according to oneembodiment of the present invention.

FIG. 36 is a diagram for one example of PS-Poll group ACK frameaccording to one embodiment of the present invention.

FIGS. 37 to 40 are diagrams to describe a channel access operation ofSTA according to one embodiment of the present invention.

FIG. 41 is a diagram for one example of a channel access methodaccording to one embodiment of the present invention.

FIG. 42 is a diagram for another example of a channel access methodaccording to one embodiment of the present invention.

FIGS. 43 to 46 are diagrams to describe a channel access type indicatingmethod according to an embodiment of the present invention.

FIGS. 47 to 50 are diagrams to describe a channel access type indicatingmethod in case of a plurality of enhanced channel access types accordingto an embodiment of the present invention.

FIG. 51 is a diagram to describe RAW assigned for a beacon interval.

FIG. 52 and FIG. 53 are diagrams for one example of a case that aprescribed STA is unable to perform PS-Poll for its own PS-Pollinterval.

FIG. 54 is a diagram for one example of a channel access method using anadditional RAW according to one embodiment of the present invention.

FIG. 55 and FIG. 56 are diagrams for one example when an additional RAWis applied.

FIG. 57 and FIG. 58 are diagrams for one example of transmitting anuplink frame to an AP from an STA for an additional RAW.

FIG. 59 is a diagram for one example of NDP UTA frame format.

FIG. 60 is a diagram for one example of transmitting a downlink dataframe to an STA from an AP for an additional RAW.

FIG. 61 is a block diagram for a configuration of a wireless deviceaccording to one embodiment of the present invention.

MODE FOR INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. In the following detailed description of the inventionincludes details to help the full understanding of the presentinvention. Yet, it is apparent to those skilled in the art that thepresent invention can be implemented without these details.

Occasionally, to prevent the present invention from getting unclear,structures and/or devices known to the public are skipped or can berepresented as block diagrams centering on the core functions of thestructures and/or devices. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

Specific terminologies used for the following description may beprovided to help the understanding of the present invention. And, theuse of the specific terminology may be modified into other forms withinthe scope of the technical idea of the present invention.

Embodiments of the present invention may be supported by the disclosedstandard documents of at least one of wireless access systems includingIEEE 802 system, 3GPP system, 3GPP LTE system, LTE-A (LTE-Advanced)system and 3GPP2 system. In particular, the steps or parts, which arenot explained to clearly reveal the technical idea of the presentinvention, in the embodiments of the present invention may be supportedby the above documents. Moreover, all terminologies disclosed in thisdocument may be supported by the above standard documents.

The following description may apply to various wireless access systemsincluding CDMA (code division multiple access), FDMA (frequency divisionmultiple access), TDMA (time division multiple access), OFDMA(orthogonal frequency division multiple access), SC-FDMA (single carrierfrequency division multiple access) and the like. CDMA can beimplemented with such a radio technology as UTRA (universal terrestrialradio access), CDMA 2000 and the like. TDMA can be implemented with sucha radio technology as GSM/GPRS/EDGE (Global System for Mobilecommunications)/General Packet Radio Service/Enhanced Data Rates for GSMEvolution). OFDMA can be implemented with such a radio technology asIEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (EvolvedUTRA), etc. UTRA is a part of UMTS (Universal Mobile TelecommunicationsSystem). 3GPP (3rd Generation Partnership Project) LTE (long termevolution) is a part of E-UMTS (Evolved UMTS) that uses E-UTRA. The 3GPPLTE adopts OFDMA in downlink (hereinafter abbreviated) DL and SC-FDMA inuplink (hereinafter abbreviated UL). And, LTE-A (LTE-Advanced) is anevolved version of 3GPP LTE.

For clarity, the following description mainly concerns IEEE 802.11system, by which the technical features of the present invention may benon-limited.

The General of System

FIG. 1 is a diagram for one example of a structure of IEEE 802.11 systemto which the present invention is applicable.

IEEE 802.11 structure may include a plurality of components and WLANsupportive of transparent STA mobility for an upper layer can beprovided by interactions of the components. A basic service set (BSS)may correspond to a basic configuration block in IEEE 802.11 LAN. FIG. 1shows one example that two basic service sets BSS 1 and BSS 2 exist andthat 2 STAs are included as members of each BSS. In particular, STA 1and STA 2 are included in the BSS 1 and STA 3 and STA 4 are included inthe BSS 2. In FIG. 1, an oval indicating the BSS can be understood asindicating a coverage area in which the STAs included in thecorresponding BSS maintain communications. This area may be named abasic service area (BSA). Once the STA moves away from the BSA, it isunable to directly communicate with other STAs within the correspondingBSA.

A BSS of a most basic type in IEEE 802.11 LAN is an independent BSS(IBSS). For instance, IBSS can have a minimum configuration including 2STAs only. Moreover, the BSS (e.g., BSS 1 or BSS 2) shown in FIG. 1,which has the simplest configuration and in which other components areomitted, may correspond to a representative example of the IBSS. Such aconfiguration is possible if STAs can directly communicate with eachother. The above-configured LAN is not configured by being designed inadvance but can be configured under the necessity of LAN. And, this maybe called an ad-hoc network.

If an STA is turned on/off or enters/escapes from a BSS area, membershipof the STA in a BSS can be dynamically changed. In order to obtain themembership in the BSS, The STA can join the BSS using a synchronizationprocedure. In order to access all services of the BSS based structure,the STA should be associated with the BSS. This association may bedynamically configured or may include a use of a DSS (distributionsystem service).

FIG. 2 is a diagram for another example of a structure of IEEE 802.11system to which the present invention is applicable. In FIG. 2,components including a distribution system (DS), a distribution systemmedium (DSM), an access point (AP) and the like are added to thestructure shown in FIG. 1.

A direct station-to-station distance in LAN may be limited by PHYperformance. This distance limit may be enough for some cases. Yet, astation-to-station communication in farther distance may be necessary insome cases. In order to support an extended coverage, a distributionsystem (DS) may be configured.

The DS means a structure in which BSSs are mutually connected to eachother. In particular, BSS may exist as a component of an extended typein a network including a plurality of BSSs instead of existingindependently as shown in FIG. 1.

The DS corresponds to a logical concept and may be specified by afeature of a distribution system medium (DSM). Regarding this, IEEE802.11 standard logically discriminates a wireless medium (WM) and adistribution system medium (DSM) from each other. Each of the logicalmedia is used for a different purpose and is also used by a differentcomponent. According to the definitions in the IEEE 802.11 standard, themedia are not limited to the same or the different. Thus, consideringthe fact that a plurality of media are logically different from eachother, the flexibility of the IEEE 802.11 LAN structure (e.g., DSstructure, other network structures, etc.) can be explained. Inparticular, the IEEE 802.11 LAN structure can be implemented intovarious examples. And, the corresponding LAN structure can be specifiedindependently by a physical property of each of the implementationexamples.

The DS can support a mobile device in a manner of providing seamlessintegration of a plurality of BSSs and logical services necessary forhandling an address to a destination.

The AP means an entity that enables associated STAs to access a DS viaWM and has STA functionality. Via the AP, data transfer between BSS andDS can be performed. For instance, STA 2 shown in FIG. 2 hasfunctionality of STA and provides a function of enabling an associatedSTA (i.e., STA 1) to access a DS. For another instance, STA 3 shown inFIG. 2 has functionality of STA and provides a function of enabling anassociated STA (i.e., STA 4) to access a DS. Since every AP basicallycorresponds to STA, it is an addressable entity. It may not be necessaryfor an address used by AP for communication on WM to be identical to anaddress used by AP for communication on DSM.

Data transmitted from one of STAs associated with an AP to an STAaddress of the AP is always received by an uncontrolled port and can beprocessed by IEEE 802.1x port access entity. Once a controlled port isauthenticated, a transmitted data (or frame) can be forwarded to a DS.

FIG. 3 is a diagram for a further example of a structure of IEEE 802.11system to which the present invention is applicable. FIG. 3conceptionally shows an extended service set (ESS) to additionallyprovide a wide coverage to the structure shown in FIG. 2.

A wireless network having an arbitrary size and complexity can beconfigured with a DS and BSSs. In IEEE 802.11 system, such a network iscalled an ESS network. The ESS may correspond to a set of BSSs connectedto a single DS. Yet, the ESS does not include the DS. The ESS network ischaracterized in looking like an IBSS network in LLC (logical linkcontrol) layer. STAs included in the ESS can communicate with each otherand mobile STAs can move away from one BSS into another BSS (within thesame ESS) in a manner of being transparent to LLC.

IEEE 802.11 assumes nothing about relatively physical locations of theBSSs shown in FIG. 3 and enables the following types. First of all, BSSscan overlap with each other in part, which is the type generally used toprovide a continuous coverage. BSSs may not be connected to each otherphysically and no limitation is put on a distance between BSSslogically. BSSs can be physically situated at the same location, whichcan be used to provide redundancy. One IBSS (or at least one IBSS) orESS networks can physically exist as one ESS network (or at least oneESS network) in the same space. This may correspond to an ESS networktype in one of a case that an ad-hoc network operates at an ESS networkexiting location, a case that IEEE 802.11 networks physicallyoverlapping with each other are configured by different organizations, acase that at least two different access and security policies arenecessary at the same location, and the like.

FIG. 4 is a diagram for one example of a structure of WLAN system. Inparticular, FIG. 4 shows one example of BSS in DS-includedinfrastructure.

In the example shown in FIG. 4, BSS 1 and BSS 2 configure an ESS. InWLAN system, STA is a device that operates by MAC/PHY regulations ofIEEE 802.11. The STA includes an AP STA and a non-AP STA. The non-AP STAgenerally corresponds to such a device directly handled by a user as alaptop, a mobile phone and the like. In the example shown in FIG. 4, STA1, STA 3 and STA 4 correspond to non-AP STAs. And, STA 2 and STA 5correspond to AP STAs.

In the following description, the non-AP STA can be called a terminal, aWireless Transmit/Receive Unit (WTRU), a User Equipment (UE), a MobileStation (MS), a Mobile Terminal, a Mobile Subscriber Station (MSS) orthe like. And, the AP includes the concept corresponding to one of aBase Station (BS), a Node-B, an evolved Node-B (eNB), a Base TransceiverSystem (BTS), a Femto BS and the like in other wireless communicationfields.

FIG. 5 is a diagram for one example of the structure of a data linklayer and a physical layer on IEEE 802.11 system to which the presentinvention is applicable.

Referring to FIG. 5, a physical layer 520 can include a PLCP entity(Physical Layer Convergence Procedure Entity) 521 and a PMD entity(Physical Medium Dependent Entity) 522. The PLCP entity 521 plays a rolein connecting a MAC sublayer 510 and a data frame to each other. The PMDentity 522 plays a role in transceiving data with at least two STAs bywireless using OFDM.

Both of the MAC sublayer 510 and the physical layer 520 can includeconceptional management entities that can be named MLME (MAC SublayerManagement Entity) 511 and PLME (Physical Layer Management Entity) 523,respectively. These entities 511 and 521 provide a layer managementservice interface through an operation of a layer management function.

In order to provide an accurate MAC operation, SME (Station ManagementEntity) 530 may exist in each user equipment. The SME 530 is amanagement entity independent from each layer and collects layer basedstate information from various layer management entities or sets valuesof specific parameters of the respective layers. The SME 530 can performsuch a function instead of general system management entities and canimplement a standard management protocol.

The above-mentioned various entities can mutually interact with eachother in various ways. In the example shown in FIG. 5, a GET/SETprimitive is exchanged. A primitive XX-GET.request is used to request avalue of MIB attribute (management information base attribute. If astate is ‘SUCCESS’, a primitive XX-GET.confirm returns a value of thecorresponding MIB attribute. In other cases, an error indication ismarked on a state field and then returned. A primitive XX-SET.request isused to make a request for setting a designated attribute as a givenvalue. If the MIB attribute means a specific operation, this requestmakes a request for executing the corresponding specific operation. If astate is ‘SUCCESS’, a primitive XX-SET.confirm means that the designatedMIB attribute is set to the requested value. In other cases, a statefield indicates an erroneous situation. If this MIB attribute means aspecific operation, the corresponding primitive can confirm that thecorresponding operation has been performed.

Referring to FIG. 5, the MLME 511 & the SME 530 and the PLME 523 & theSME 530 can exchange various primitives through MLME_SAP (MLME_ServiceAccess Point) 550 and PLME_SAP (PLME_Service Access Point) 560,respectively. And, the MLME 511 and the PLME 523 can exchange primitivesthrough MLME-PLME_SAP (MLME-PLME_Service Access Point) 570.

Link Setup Process

FIG. 6 is a diagram to describe a general link setup process in a WLANsystem to which the present invention is applicable.

In order for an STA to transceive data by setting up a link with anetwork, the STA should discover a network, perform authentication,establish association, perform an authentication procedure for security,and the like. A link setup process can be named a session initiationprocess or a session setup process. And, the discovery, authentication,association and security setup steps of the link setup process can becommonly named an association process.

One example of a link setup process is described with reference to FIG.6 as follows.

In a step S610, an STA can perform a network discovery action. Thenetwork discovery action can include a scanning action of the STA. Inparticular, in order to access the network, the STA should discover ajoinable network. The STA needs to identify a compatible network beforejoining a wireless network. In doing so, a process for identifying anetwork existing in a specific area is called a scanning.

The scanning can be categorized into an active scanning or a passivescanning.

FIG. 6 shows a network discovery action including an active scanningprocess. In the active scanning, an STA performing a scanning transmitsa probe request frame for searching what kind of AP exists nearby whileswitching channels and then waits for a response to the transmittedprobe request frame. A responder transmits a probe response frame inresponse to the probe request frame to the STA having transmitted theprobe request frame. In this case, the responder may include an STAhaving transmitted a beacon frame last in a BSS of a scanned channel. Inthe BSS, since an AP transmits the beacon frame, the AP becomes theresponder. In IBSS, since each of STAs within the IBSS transmits thebeacon frame in turn, the responder is not fixed. For instance, if anSTA transmits a probe request frame on channel #1 and then receives aprobe response frame on the channel #1, the STA saves BBS relatedinformation contained in the received probe response frame and is thenable to perform a scanning in the same manner by switching to a nextchannel (e.g., channel #2) [i.e., transmission of a probe request onchannel #2 and reception of a probe response on channel #2].

The scanning action may be performed by the passive scanning scheme [notshown in FIG. 6]. In the passive scanning, an STA performing thescanning waits for a beacon frame while switching channels. The beaconframe is one of management frames in IEEE 802.11 and is periodicallytransmitted in order to announce an existence of a wireless network andto enable an STA performing a scanning to discover and join thecorresponding wireless network. In a BSS, an AP plays a role intransmitting a beacon frame periodically. In an IBSS, each of STAswithin the IBSS transmits a beacon frame in turn. If an STA performing ascanning receives a beacon frame, the corresponding STA saves aninformation on a BSS included in the beacon frame and then records abeacon frame information on each channel while switching to anotherchannel. Having received the beacon frame, the STA saves a BSS relatedinformation contained in the received beacon frame and is then able toperform a scanning on a next channel by switching to the next channel.

Comparing an active scanning and a passive canning to each other, theactive scanning is more advantageous than the passive scanning in delayand power consumption.

After the STA has discovered the network, an authentication process canbe performed in a step S620. This authentication process can be named afirst authentication process to be clearly discriminated from a securitysetup action in a step S640 described later.

The authentication process includes a following process. First of all,the STA transmits an authentication request frame to the AP. Secondly,the AP transmits an authentication response frame to the STA in responseto the authentication request frame. The authentication frame used forthe authentication request/response corresponds to a management frame.

The authentication frame can contain informations on an authenticationalgorithm number, an authentication transaction sequence number, astatus code, a challenge text, an RSN (robust security network), afinite cyclic group, and the like. These informations correspond to someexample of informations containable in the authenticationrequest/response frame, can be substituted with other information, andmay further include additional informations.

The STA can transmit an authentication request frame to the AP. Based onthe information contained in the received authentication request frame,the AP can determine whether to allow the authentication of thecorresponding STA. The AP is able to provide a result of theauthentication processing to the STA through an authentication responseframe.

After the STA has been successfully authenticated, an associationprocess can be performed in a step S630. The association processincludes a following process. First of all, the STA transmits anassociation request frame to the AP. Secondly, the AP transmits anassociation response frame to the STA in response to the associationrequest frame.

For instance, the association request frame can include informationsrelated to various capabilities, e.g., informations on a beacon listeninterval, a service set identifier (SSID), supported rates, supportedchannels, an RSN, a mobility domain, supported operating classes, a TIM(traffic indication map) broadcast request, an interworking servicecapability and the like.

For instance, the association response frame can include informationsrelated to various capabilities, e.g., informations on a status code, anAID (association ID), supported rates, an EDCA (enhanced distributedchannel access) parameter set, an RCPI (received channel powerindicator), an RSNI (received signal to noise indicator), a mobilitydomain, a timeout interval (association comeback time), an overlappingBSS scan parameter, a TIM broadcast response, a QoS (quality of service)map and the like.

These informations correspond to some example of informationscontainable in the authentication request/response frame, can besubstituted with other information, and may further include additionalinformations.

After the STA has been successfully associated with the network, asecurity setup process can be performed in a step S640. The securitysetup process in the step S640 may be called an authentication processthrough RSNA (robust security network association) request/response. Theauthentication process of the step S620 may be named a firstauthentication process, while the security setup process of the stepS640 may be simply named an authentication process.

The security setup process of the step S640 can include a private keysetup process by 4-way handshaking through EAPOL (extensibleauthentication protocol over LAN) for example. And, the security setupprocess can be performed by a security scheme that is not defined inIEEE 802.11 Standard.

Evolution of WLAN

IEEE 802.11n exists as a technology standard stipulated relativelyrecently in order to overcome the limits put on a communication speed ina wireless LAN. The objects of IEEE 802.11n are to increase a speed andreliability of a network and to extend an operating distance of awireless network. In particular, IEEE 802.11n supports high throughput(HT) of which data processing speed is equal to or greater than maximum540 Mbps. In order to minimize transmission error and optimize a dataspeed or rate, IEEE 802.11n is based on MIMO (multiple inputs andmultiple outputs) technology that uses multiple antennas for atransmitting unit end and a receiving end unit both.

As WLAN is supplied widely and actively and applications using WLAN arediversified, the necessity for a new WLAN system to support a throughputhigher than a data processing speed supported by IEEE 802.11n isincreasingly rising. A next generation WLAN system supportive of VHT(very high throughput) is a next version (e.g., IEEE 802.11ac) of IEEE802.11n WLAN system and corresponds to one of IEEE 802.11 WLAN systemsproposed recently and newly to support a data processing sped over 1Gbps at a MAC service access point (SAP).

A next WLAN system supports a transmission of MU-MIMO (multi usermultiple input multiple output) for enabling a plurality of STAs toaccess a channel simultaneously in order to efficiently use wirelesschannels. According to MU-MIMO transmission scheme, an AP is able tosimultaneously transmit a packet to at least one or more MIMO-pairedSTAs. And, there has been much discussion about supporting a WLAN systemoperation on a whitespace. For instance, the introduction of a WLANsystem on a TV whitespace (ES) such as a frequency band (e.g., 54˜698MHz band) in idle state due to the digitalization of analog TV has beendiscussed as IEEE 802.11af Standard. Yet, this is just one example. Thewhitespace can be regarded as a licensed band that can be incumbentlyused by a licensed user. In this case, the licensed user means a userthat is licensed to use a licensed band. And, the licensed user can becalled one of a licensed device, a primary user, an incumbent user andthe like.

For instance, an AP and/or STA operation on WS should provide aprotection function for a licensed user. For instance, in case that alicensed user such as a microphone is already using a specific WSchannel corresponding to a frequency band divided on regulation to havea specific bandwidth on a WS band, an AP and/or STA is unable to use thefrequency band amounting to the corresponding WS channel to protect thelicensed user. If a licensed user uses a frequency band currently usedfor a current frame transmission and/or reception, an AP and/or STAshould stop using the corresponding frequency band.

Hence, the AP and/or STA should precedently perform a procedure forchecking whether a use of a specific frequency band within a WS band isavailable, i.e., whether a licensed user exists on the frequency band.Checking whether the licensed user exists on the specific frequency bandis called a spectrum sensing. As a spectrum sensing mechanism, one ofenergy detection, signature detection and the like is utilized. If astrength of a received signal is equal to or greater than apredetermined value, it is able to determine that the licensed usercurrently uses the specific frequency band. If a DTV preamble isdetected, it is able to determine that the licensed user currently usesthe specific frequency band.

M2M (machine-to-machine) communication technology is currently discussedas a next generation communication technology. In IEEE 802.11 WLANsystem, a technology standard for supporting M2M communication isdeveloped as IEEE 802.11ah. The M2M communication means a communicationsystem that includes at least one machine and may be called MTC (machinetype communication) or the like. In this case, ‘machine’ means an entitythat does not require direct human manipulation or intervention. Forinstance, a device such as a wireless communication module installedmeter and a wireless communication module installed auto vending machinemay correspond to one example of a machine as well as a user device suchas a smartphone that can perform a communication by automaticallyaccessing a network without user's manipulation/intervention. The M2Mcommunication can include one of a communication between devices (e.g.,a D2D (device-to-device) communication), a communication between adevice and a server (e.g., an application server), and the like. As oneexample of the device-to-server communication, there is a communicationbetween an auto vending machine and a server, a communication between aPOS (point of sale) device and a server, a communication between anelectricity/gas/water meter and a server, or the like. Besides, M2Mcommunication based applications can include security, transportation,health case and the like. Considering the properties of the applicationexamples, M2M communication should be generally able to supporttransmission/reception of a small amount of data occasionally in anenvironment in which many devices exist.

In particular, M2M communication should be able to support a largenumber of STAs. Although a currently defined WLAN system assumes a casethat maximum 207 STAs are associated with a single AP, methods forsupporting a case that a number of STAs more than 2007 STAs areassociated with a single AP are currently discussed in M2Mcommunication. Moreover, in M2M communication, it is estimated thatthere will be many applications that support/require a low transmissionspeed. In order to support this smoothly, for instance, in WLAN system,an STA is able to recognize a presence or non-presence of data, which isto be transmitted to the STA, based on TIM (traffic indication map)element. And, methods for reducing a bitmap size of TIM are currentlydiscussed. Moreover, in M2M communication, it is estimated that therewill be many traffics that have a considerably longtransmission/reception interval. For instance, like anelectricity/gas/water used amount, it is required to transceive aconsiderably small amount of data in each long periodicity (e.g., 1month, etc.). Hence, although the number of STAs associable with asingle AP increases highly, methods for efficiently supporting a casethat the number of STAs having a data frame supposed to be received froman AP in a single beacon periodicity is considerably small are currentlydiscussed.

Thus, the WLAN technology is evolving fast and technologies for a directlink setup, an enhancement of media streaming performance, support of afast and/or large-scale initial session setup, support of an extendedbandwidth and operating frequency, and the like are currently developed.

Frame Structure

FIG. 7 shows one example of a MAC frame format of IEEE 802.11 system towhich the present invention is applicable.

Referring to FIG. 7, a MAC frame format includes a MAC header (MHR), aMAC payload and a MAC footer (MFR). The MHR is defined as a regionincluding a frame control field, a duration/identifier (duration/ID)field, an address 1 field, an address 2 field, an address 3 field, asequence control field, an address 4 field, a QoS control field, and anHT control field. A frame body field is defined as a MAC payload. Datadesired to be transmitted by an upper layer is located in the frame bodyfield. And, the frame body field has a variable size. A frame checksequence (FCS) field is defined as a MAC footer and is used for an errorsearch of a MAC frame.

The first 3 fields (i.e., the frame control field, the duration/IDfield, and the address 1 field) configure a minimum frame format andexist in all frames. And, other fields can exist in a specific frametype only.

Informations included in the respective fields mentioned in the abovedescription can follow the definition of IEEE 802.11 system. Therespective fields mentioned in the foregoing description correspond toexamples of the fields that can be included in the MAC frame, may besubstituted with other fields, or may further include additional fields.

FIG. 8 shows one example of HT format of the HT control field in the MACframe according to FIG. 7.

Referring to FIG. 8, the HT control field may include a VHT subfield, alink adaptation subfield, a calibration position subfield, a calibrationsequence subfield, a CSI/Steering (channel state information/steering)subfield, an NDP (null data packet) announcement subfield, an AC (accesscategory) constraint subfield, an RDG/More PPDU (reverse directiongrant/More PPDU) subfield, a reserved subfield, and the like.

The link adaption subfield can include a TRQ (training request)subfield, an MAI [MCS (modulation and coding scheme) request or ASEL(antenna selection) indication] subfield, an MFSI (MCS feedback sequenceidentifier) subfield, an MFB/ASELC (MCS feedback and antenna selectioncommand/data) subfield, and the like.

If a request for a sounding PPDU transmission is made to a responder,the TRQ subfield is set to 1. If a request for a sounding PPDUtransmission is not made to a responder, the TRQ subfield is set to 0.If the MAI subfield is set to 14, it means an antenna selection (ASEL)indication and the MFB/ASELC subfield is interpreted as antennaselection command/data. Otherwise, the MAI subfield indicates an MCSrequest and the MFB/ASELC subfield is interpreted as an MCS feedback.When the MAI subfield indicates an MCS request (MRQ), if any MCSfeedback is not requested, the MAI subfield is set to 0. If the MCSfeedback is requested, the MAI subfield is set to 1. The sounding PPDUmeans PPDU that carries a training symbol usable for channel estimation.

The respective subfields mentioned in the above description correspondto examples of the subfields that can be included in the HT controlfield, may be substituted with other subfields, or may further includeadditional subfields.

FIG. 9 shows one example of VHT format of the HT control field in theMAC frame according to FIG. 7.

Referring to FIG. 9, the HT control field may include a VHT subfield, anMRQ subfield, an MSI subfield, an MCS feedback sequence indication/groupID least significant bit (MFSI/GID-L (LSB of Group ID)) subfield, an MFBsubfield, a group ID most significant bit (GID-H (MSB of Group ID))subfield, a coding type subfield, an MFC response transmission type (FBTx Type: Transmission type of MFB response) subfield, an unsolicited MFBsubfield, an AC constraint subfield, an RDG/More PPDU subfield, and thelike. And, the MFB subfield may include a VHT N_STS (Number of spacetime streams) subfield, an MCS subfield, a BW (bandwidth) subfield, anSNR (Signal to Noise Ratio) subfield, and the like.

Table 1 shows descriptions of the respective subfields in the VHT formatof the HT control field.

TABLE 1 Subfield Meanings Definitions MRQ MCS request This is set to 1if an MCS feedback (solicited MFB) is requested. Otherwise, this is setto 0. MSI MRQ sequence If MRQ subfield is set to 1, MSI subfieldincludes a identifier sequence number ranging from 0 to 6. If MRQsubfield is set to 0, MSI subfield is reserved. MFSI/GID-L MFB sequenceOf unsolicited) MFB subfield is set to 0, MFSI/GID-L identifier/LSBsubfield includes a reception value of MSI included in of Group ID theframe indicated by MFB information. If unsolicited MFB subfield is setto 1, MFSI/GID-L subfield includes 3 LSBs of group ID of PPDU indicatedby unsolicited MFB. MFB VHT N_STS, MFB subfield indicates recommendedMF. If MCS = 15 MCS, BW, SNR and VHT N_STS = 7, it indicates that nofeedback exists. feedback GID-H MSB of Group If unsolicited MFB subfieldis set to 1, GID-H subfield ID includes 3 MSBs of group ID of PPDUindicated by unsolicited MFB. Coding Type Coding type of If unsolicitedMFB subfield is set to 1, coding type MFB response subfield includescoding information (e.g., 1 in case of BCC (binary convolutional code),0 in case of LDPC (low-density parity check). Otherwise, this subfieldis reserved. FB Tx Type Transmission If unsolicited MFB subfield is setto 1 and FB Tx Type type of MFB subfield is set to 0, unsolicited MFBindicates one of a response transmit diversity using unbeamformed VHTPPDU and a transmit diversity using STBC (space-time block coding) VHTPPDU. If unsolicited MFB subfield is set to 1 and FB Tx Type subfield isset to 1, unsolicited MFB indicates beamformed SU-MIMO (Single UserMIMO) VHT PPDU 

 . Otherwise, this subfield is reserved. Unsolicited Unsolicited If MFBis not a response to MRQ, this subfield is set to MFB MCS feedback 1. IfMFB is a response to MRQ, this subfield is set to 0. indicator ACConstraint If a response to a reverse direction grant (RDG) includes adata frame from a prescribed TID (traffic identifier), this subfield isset to 0. If a response to a reverse direction grant (RDG) includes aframe from the same AC of a last data frame received from a same reversedirection (RD) initiator), this subfield is set to 1. RDG/More IfRDG/More PPDU subfield is set to 0, it indicates that PPDU there is noreverse direction grant (RDG) in case of a transmission from a reversedirection (RD) initiator or that PPDU carrying MAC frame is a finaltransmission in case of a transmission from a reverse direction (RD)responder. If RDG/More PPDU subfield is set to 1, it indicates that areverse direction grant (RDG) exists in case of a transmission from areverse direction (RD) initiator or that PPDU carrying a MAC frame isfollowed by another PPDU in case of a transmission from a responder.

The respective subfields mentioned in the above description correspondto examples of the subfields that can be included in the HT controlfield, may be substituted with other subfields, or may further includeadditional subfields.

Meanwhile, a MAC sublayer delivers a MAC protocol data unit (MPDU) to aphysical layer as a physical (PHY) service data unit (PSDU). A PLCPentity generates PLCP protocol data unit (PPDU) by attaching a physical(PHY) header and a preamble to the received PSDU.

FIG. 10 shows one example of PPDU frame format of IEEE 802.11n system towhich the present invention is applicable.

FIG. 10 (a) shows one example of PPDU frames according to a non-HTformat, an HT mixed format and an HT-greenfield format.

The non-HT format indicates a frame format for an existing legacy system(IEEE 802.11a/g) STA. The non-HT format PPDU includes a legacy formatpreamble consisting of L-STF (Legacy-Short Training field), L-LTF(Legacy-Long Training field) and L-SIG (Legacy-Signal) field.

The HT mixed format allows a communication of an existing legacy systemSTA and also indicates a frame format for IEEE 802.11n STA. The HT mixedformat PPDU includes a legacy format preamble consisting of L-STF, L-LTFand L-SIG and an HT format preamble consisting of HT-STF (HT-ShortTraining field), HT-LTF (HT-Long Training field) and HT-SIG (HT-Signal)field. Since the L-STF, L-LTF and L-SIG mean the legacy fields forbackward compatibility), a configuration from L-STF to L-SIG isidentical to that of the non-HT format. And, an STA is able to recognizethe mixed format PPDU using a following HT-SIG field.

The HT-Greenfield format is not compatible with an existing legacysystem and indicates a frame format for IEEE 802.11n ST. TheHT-Greenfield format PPDU includes a greenfield preamble consisting ofHT-GF-STF (HT-Greenfield-STF), HT-LTF1, HT-SIG and at least one HT-LTF.

The data field includes a SERVICE field, PSDU, tail bit, and pad bit.All bots of the data field are scrambled.

FIG. 10 (b) shows a service field included in the data field. Theservice field retains 16 bits. The bits are numbered by 0 to 15. And,the bits are sequentially transmitted by starting with the bit #0. Thebits #0 to #6 are set to 0 and used to synchronize a descrambler withina receiving end.

FIG. 11 shows one example of VHT PPDU frame format of IEEE 802.11acsystem to which the present invention is applicable.

Referring to FIG. 11, VHT format PPDU includes a legacy format preambleconsisting of L-STF, L-LTF and L-SIG and a VHT format preambleconsisting of VHT-SIG-A, HT-STF and HT-LTFs before a data field. Sincethe L-STF, L-LTF and L-SIG mean the legacy fields for backwardcompatibility, a configuration from the L-STF to the L-SIG is identicalto that of the non-HT format and an STA is able to recognize the VHTformat PPDU using a following VHT-SIG field.

The L-STF is the field for frame detection, AGC (Auto Gain Control),diversity detection, coarse frequency/time synchronization, and thelike. The L-LTF is the field for fine frequency/time synchronization,channel estimation and the like. The L-SIG is the field for legacycontrol information transmission. The VHT-SIG-A is the VHT field forcommon control information transmission of VHT STAs. The VHT-STF is thefield for AGC for MIMO and a beamformed stream. The VHT-LTFs is thefield for channel estimation for MIMO and a beamformed stream. And, theVHT-SIG-B is the field for transmitting a control information specifiedfor each STA.

Medium Access Mechanism

In WLAN system according to IEEE 802.11, a basic access mechanism of MAC(medium access control) is a CSMA/CA (carrier sense multiple access withcollision avoidance) mechanism. The CSMA/CA mechanism may be called DCF(distributed coordination function) of IEEE 802.11 MAC and basicallyemployees an access mechanism ‘listen before talk’. According to anaccess mechanism of such a type, before initiating a transmission, an APand/or STA can perform CCA (clear channel assessment) for sensing aradio channel or medium during a prescribed time interval (e.g., DIFS(DCF inter-frame space). As a result of the sensing, if it is determinedthat a medium is in idle status, the AP and/or STA starts a frametransmission through a corresponding medium. On the contrary, if it isdetected that a medium is in occupied status, the corresponding APand/or STA sets up a delay interval (e.g., a random backoff period) fora medium access instead of starting its own transmission, stands by, andis then able to attempt a frame transmission. Since several STAs areexpected to attempt frame transmission after standbys for differenttimes owing to the application of the random backoff period, it is ableto minimize collision.

IEEE 802.11 MAC protocol provides HCF (hybrid coordination function).The HCF is based on the DCF and PCF (point coordination function). ThePCF corresponds to a polling-based synchronous access scheme and means ascheme of performing polling periodically in order for all receiving APsand/or STAs to receive data frame. The HCF has EDCA (enhanceddistributed channel access) and HCCA (HCF controlled channel access).The EDCA uses a contention based access scheme for a provider to providea data frame to multiple users. And, the HCCA uses a non-contentionbased channel access scheme using a polling mechanism. The HCF includesa medium access mechanism for improving QoS (quality of service) of WLANand is able to transmit QoS data in both a contention period (CP) and acontention free period (CFP).

FIG. 12 is a diagram to describe a backoff process in a WLAN system towhich the present invention is applicable.

An operation based on a random backoff period is described withreference to FIG. 12 as follows.

First of all, if a medium in occupied or busy status enters an idlestatus, several STAs can attempt data (or frame) transmission. In doingso, according to a scheme of minimizing collision, each of the STAsselects a random backoff count, stands by in a slot time amounting tothe selected random backoff count, and is then able to attempt thetransmission. The random backoff count has a pseudo-random integer valueand can be determined as 0 or one of values in a CW range. In this case,the CW is a contention window parameter value. CWmin is given as aninitial value to the CW parameter. Yet, if the transmission fails [e.g.,ACK for a transmitted frame is not received], the CW parameter can takea doubled value. If the CW parameter value becomes CWmax, the datatransmission can be attempted by maintaining the CWmax value until thedata transmission becomes successful. If the data transmission issuccessfully completed, the CW parameter value is rest to the CWminvalue. Preferably, a value of each of the CW, CWmin and CWmax is set to(2n−1), where n=0, 1, 2 . . . .

If a random backoff process starts, the STA keeps monitoring a mediumwhile a backoff slot is counted down according to the determined backoffcount value. If the STA monitors that the medium is in a busy status,the STA waits by stopping the countdown. If the medium enters the idlestatus, the STA resumes the remaining countdown.

In the example shown in FIG. 12, in case that a packet to be transmittedarrives at the MAC of STA3, the STA3 confirms that the medium is in idlestatus and is then able to directly transmit a frame. Meanwhile, therest of the STAs monitor that the medium is in busy status and standsby. In doing so, since data to be transmitted may be generated from eachof STA1, STA2 and STA5, each of the STAs stands by for DIFS ifmonitoring that the medium is in idle status and is then able to countdown a backoff slot according to a random backoff count value selectedby itself. In the example shown in FIG. 17, the STA2 selects a smallestbackoff count value and the STA1 selects a biggest backoff count value.In particular, FIG. 17 shows one example that a residual backoff time ofthe STA5 is shorter than that of the STA1 at the timing point at whichthe STA2 finishes the backoff count and starts a frame transmission.Each of the STA1 and the STA5 stops the countdown temporarily and standsby, while the STA2 occupies the medium. As the occupation by the STA2 isended, if the medium enters the idle status again, each of the STA1 andthe STA5 stands by for DIFS and then resumes the paused backoff count.In particular, the frame transmission can be started after the rest ofbackoff slots amounting to the residual backoff time have been counteddown. Since the residual backoff time of the STA5 is shorter than thatof the STA1, the STA5 starts the frame transmission. Meanwhile, whilethe STA2 occupies the medium, data can be generated from the STA4. Indoing so, from the viewpoint of the STA4, if the medium enters an idlestatus, the STA4 stands by for DIFS, performs a countdown according to arandom backoff count value selected by itself, and is then able to starta frame transmission. FIG. 12 shows one example of a case that aresidual backoff time of the STA5 accidently coincides with a randombackoff count value of the STA4. In this case, collision may occurbetween the STA4 and the STA5. In case that the collision occurs, eachof the STA4 and the STA5 is unable to receive ACK and fails in the datatransmission. In this case, each of the STA4 and the STA5 doubles a CWvalue, selects a random backoff count value, and is then able to performa countdown. Meanwhile, the STA1 stands by while the medium is in theoccupied (or busy) status due to the transmissions by the STA4 and theSTA5. If the medium enters an idle status, the STA1 stands by for DIFS.If the residual backoff time elapses, the STA1 is able to start theframe transmission.

Sensing Operation of STA

As mentioned in the foregoing description, the CSMA/CA mechanismincludes a virtual carrier sensing as well as a physical carrier sensingfor an AP and/or STA to directly sense a medium. The virtual carriersensing is provided to complement such a problem, which may be generatedfrom a medium access, as a hidden node problem and the like. For thevirtual carrier sensing, MAC of WLAN system is able to use a networkallocation vector (NAV). The NAV is a value for an AP and/or STAcurrently using a medium or having an authority to use to indicate atime, which is left until a medium enters an available status, toanother AP and/or STA. Hence, the value set as the NAV corresponds to aperiod scheduled for an AP and/or STA transmitting a corresponding frameto use a medium. If an STA receives the NAV value, the STA is prohibitedfrom a medium access during the corresponding period. For instance, theNAV can be set according to a value of a duration field of a MAC headerof a frame.

In order to reduce possibility of collision, a robust collisiondetecting mechanism has been introduced. This shall be described withreference to FIG. 18 and FIG. 19. Although a carrier sensing range and acarrier transmission range may not be actually identical to each other,assume that the two ranges are identical to each other for clarity ofthe following description.

FIG. 13 is a diagram to describe a hidden node and an exposed node.

FIG. 13 (a) shows one example of a hidden node, which corresponds to acase that STA C has an information to transmit in the course of acommunication between STA A and STA B. In particular, despite asituation that the STA is transmitting an information to the STA B, theSTA C can determine that a medium is in idle status when the STA Cperforms a carrier sensing before sending data to the STA B. The reasonfor this is that a transmission (i.e., a medium occupation) by the STA Amay not be sensed at a location of the STA C. In this case, since theSTA B receives both information of the STA A and information of the STAC simultaneously, a collision occurs. In doing so, the STA A can becalled a hidden node of the STA C.

FIG. 13 (b) shows one example of an exposed node, which corresponds to acase that STA C has an information to transmit to STA D in a situationthat STA B is transmitting data to STA A. In doing so, if the STA Cperforms a carrier sensing, it is able to determine that a medium isoccupied due to the transmission by the STA B. Hence, although the STA Chas the information to transmit to the STA D, since the medium occupiedstatus is sensed, the STA C should stand by until the medium enters anidle status. Yet, since the STA A is actually located out of atransmission range of the STA C, the transmission from the STA C and thetransmission from the STA B may not collide with each other from theviewpoint of the STA A, the STA C may stand by unnecessarily until theSTA B stops the transmission. In doing so, the STA C can be called anexposed node of the STAB.

FIG. 14 is a diagram to describe RTS and CTS.

First of all, in order to efficiently use a collision avoidancemechanism in the exemplary situation shown in FIG. 13, it is able to usesuch a short signaling packet as RTS (request to send), CTS (clear tosend) and the like. In order to enable neighbor STA(s) to overhear,RTS/CTS between two STAs can be set to enable the neighbor STA(s) toconsider whether to perform an information transmission between the twoSTAs. For instance, if a data transmitting STA transmits an RTS frame toa data receiving STA, the data receiving STA is able to announce that itwill receive data by transmitting a CTS frame to neighbor userequipments.

FIG. 14 (a) shows one example of a method of solving a hidden nodeproblem, which assumes a case that both STA A and STA C intend totransmit data to STA B. if the STA A sends RTS to the STA B, the STA Btransmits CTS to both of the STA A and the STA C neighboring to the STAB. As a result, the STA C stands by until the data transmission betweenthe STA A and the STA B ends, whereby collision can be avoided.

FIG. 14 (b) shows one example of a method of solving an exposed nodeproblem. As STA C overhears RTS/CTS transmission between STA A and STAB, the STA C can determine that collision will not occur despite thatthe STA C transmits data to another STA (e.g., STA D). In particular,the STA B transmits RTS to all neighbor user equipments and the STA Ahaving data to send actually transmits CTS only. Since the STA Creceives the RTS but fails in receiving the CTS of the STA A, the STA Ccan recognize that the STA A is out of a carrier sensing of the STA C.

IFS (Inter-Frame Space)

A time space between two frames is defined as IFS (inter-frame space).STA determines whether a channel is used for IFS through a carriersensing. DCF MAC layer defines 4 kinds of inter-frame spaces (IFSs), bywhich a priority of occupying a radio medium is determined.

IFS is set to a specific value depending on a physical layerirrespective of a bit rate of STA. Types of IFS include SIFS (ShortIFS), PIFS (PCF IFS), DIFS (DCF IFS), and EIFS (Extended IFS). SIFS(Short IFS) is used for a transmission of RTS/CTS and a transmission ofACK frame and has a top priority. PIFS (PCF IFS) is used for PCF frametransmission. DIFS (DCF IFS) is used for DCF frame transmission. EIFS(Extended IFS) is used for an occurrence of frame transmission erroronly and does not have a fixed interval.

Relation between IFSs is defined as a time gap on a medium and relatedattributes are provided by a physical layer, as shown in FIG. 15.

FIG. 15 is a diagram for one example of relation of IFS.

In every medium timing, an end timing point of a last symbol of PPDUindicates a transmission end and a first symbol of a preamble of a nextPPDU indicates a transmission start. Every MAC timing can be determinedwith reference to PHY-TXEND.confirm primitive, PHYTXSTART.confirmprimitive, PHY-RXSTART.indication primitive, and PHY-RXEND.indicationprimitive.

Referring to FIG. 15, SIFS time (aSIFSTime) and slot time (aSlotTime)can be determined per physical layer. The SIFS time has a fixed valueand the slot time can be dynamically changed in accordance with a changeof an air propagation time (aAirPropagationTime). The SIFS time and theslot time are defined as Formula 1 and Formula 2, respectively.

aSIFSTime=aRxRFDelay+aRxPLCPDelay+aMACProcessingDelay+aRxTxTurnaroundTime  [Formula1]

aSlotTime=aCCATime+aRxTxTurnaroundTime+aAirPropagationTime+aMACProcessingDelay  [Formula2]

PIFS and SIFS are defined as Formula 3 and Formula 4, respectively.

PIFS=aSIFSTime+aSlotTime  [Formula 3]

DIFS=aSIFSTime+2*aSlotTime  [Formula 4]

EIFS is calculated from SIFS, DIFS and ACK transmission time (ACKTxTime)by Formula 5. The ACK transmission time (ACKTxTime) is expressed asmicroseconds required for a transmission of ACK frame including apreamble in a lowest physical layer mandatory rate, a PLCP header andadditional physical layer dependent informations.

EIFS=aSIFSTime+DIFS+ACKTxTime  [Formula 5]

SIFS, PIFS and DIFS exemplarily shown in FIG. 15 are measured on MACslot boundaries (TxSIFS, TxPIFS, TxDIFS) different from a medium. Such aslot boundary is defined as a time at which a transmitter is turned onby a MAC layer in order to match different IFS timings after detectionof CCA result of a previous slot time. The MAC slot boundaries for SIFS,PIFS and DIFS are defined as Formulas 6 to 8, respectively.

TxSIFS=SIFS−aRxTxTurnaroundTime  [Formula 6]

TxPIFS=TxSIFS+aSlotTime  [Formula 7]

TxDIFS=TxSIFS+2*aSlotTime  [Formula 8]

Power Management

As mentioned in the foregoing description, in WLAN system, STA shouldperform a channel sensing before performing transmission/reception. Yet,sensing a channel all the time requires a consistent power consumptionof the STA. there is no big difference between a power consumption inreception status and a power consumption in transmission status. And,keeping the reception status puts a burden on a power-limited STA (i.e.,a battery-operable STA). Hence, if an STA maintains a reception standbystatus in order to consistently sense a channel, it consumes a powerinefficiently without special advantages in aspect of WLAN throughput.In order to solve this problem, a WLAN system supports a powermanagement (PM) mode of STA.

The power management mode of STA can be divided into an active mode anda power save mode. The STA basically operates in active mode. The STAoperating in active mode maintains an awake state. The awake state meansa state in which a normal operation such as a frame transceiving, achannel scanning and the like is possible. On the other hand, the STAoperating in PS mode operates in a manner of switching between a sleepstate and an awake state. The STA operating in sleep state operates witha minimum power but does not perform a channel scanning as well as aframe transceiving.

Since a power consumption decreases if an STA operates in sleep state aslong as possible, an operating period of the STA increases. Yet, since aframe transceiving is impossible in the sleep state, the STA is unableto operate long unconditionally. If there is a frame an STA operating insleep state will transmit to an AP, the STA can transmit a frame byswitching to an awake state. On the contrary, if there is no frame theAP will transmit to the STA, the STA in the sleep state is unable toreceive the frame and is also unable to recognize a presence of theframe to receive. Hence, the STA may need an operation of switching toan awake state in accordance with specific periodicity in order torecognize a presence or non-presence of a frame to be transmitted to thecorresponding STA (or, in order to receive the frame if the frame ispresent).

FIG. 16 is a diagram to describe a power management operation.

Referring to FIG. 16, an AP 210 transmits beacon frames to STAs in a BSSby predetermined periods [S211, S212, S213, S214, S215, S216]. In thebeacon frame, a TIM (traffic indication map) information element iscontained. The TIM information element contains an information for theAP 210 to indicate that there is a buffered traffic for STAs associatedwith the AP 210 and that the AP 210 will transmit a frame. TIM elementmay include a TIM used to indicate a unicast frame and a DTIM (deliverytraffic indication map) used to indicate a multicast or broadcast frame.

The AP 210 can transmit the DTIM once per 3 transmissions of the beaconframes.

STA1 220 and STA2 230 are STAs operating in PS mode. Each of the STA1220 and the STA2 230 can be set to receive the TIM element transmittedby the AP 210 by switching to an awake state from a sleep state in everywakeup interval of prescribed periodicity. Each of the STAs cancalculate a timing point of switching to an awake state based on itslocal clock. In the example shown in FIG. 20, assume that the clock ofthe STA coincides with a clock of the AP.

For instance, the prescribed wakeup interval can be set for the STA1 220to receive the TIM element by switching to the awake state in everybeacon interval. Hence, when the AP 210 transmits the beacon frame forthe 1^(st) time [S211], the STA1 220 can switch to the awake state[S221]. The STA1 220 receives the beacon frame and is able to acquirethe TIM element. If the acquired TIM element indicates that there is aframe to be transmitted to the STA1 220, the STA1 220 can transmit aPS-Poll (Power Save-Poll) frame, which is provided to make a request fora frame transmission to the AP 210, to the AP 210 [S221 a]. The AP 210is able to transmit a frame to the STA1 220 in response to the PS-Pollframe [S231]. Having received the frame, the STA1 220 operates byswitching to the sleep state again.

When the AP 210 transmits the beacon frame for the 2^(nd) time, since amedium is occupied (i.e., the medium is a busy medium) in a manner thatanother device accesses the medium for example, the AP 210 is unable totransmit the beacon frame to correspond to an accurate beacon intervalbut is able to transmit the beacon frame at a delayed timing point[S212]. In this case, although the STA1 220 switches its operating modeto the awake state to correspond to the beacon interval, since the STA1220 fails in receiving the beacon frame transmitted by being delayed,the STA1 220 switches to the sleep state again [S222].

When the AP 210 transmits the beacon frame for the 3^(rd) time, TIMelement set as DTIM may be contained in the corresponding beacon frame.Yet, since the medium is occupied (i.e., the medium is a busy medium),the AP 210 transmits a delayed beacon frame [S213]. The STA1 220operates by switching to the awake state to correspond to the beaconinterval and is able to acquire DTIM through the beacon frametransmitted by the AP 210. The DTIM acquired by the STA1 220 is assumedas indicating that there is no frame to be transmitted to the STA1 220and that a frame for another STA is present. In this case, the STA1 220confirms that there is no frame to receive and is then able to operateby switching to the sleep state again. After transmitting the beaconframe, the AP 210 transmits a frame to the corresponding STA [S232].

The AP 210 transmits the beacon frame for the 4^(th) time [S214]. Yet,since the STA1 220 is unable to acquire information, which indicatesthat a buffered traffic for the STA1 220 is present, through the 2previous TIM element receptions, the STA1 220 is able to adjust a wakeupinterval for the TIM element reception. On the other hand, if asignaling information for adjusting a wakeup interval value of the STA1220 is contained in the beacon frame transmitted by the AP 210, thewakeup interval value of the STA1 can be adjusted. According to thepresent example, the STA1 220 can be set to switch an operating state ina manner that the STA1 220 wakes up once in every 3 beacon intervalsinstead of switching the operating state for the TIM element receptionin every beacon interval. Hence, since the STA1 220 maintains the sleepstate at the timing point at which the AP 210 transmits the beacon framefor the 5^(th) time [S215] after transmitting the 4^(th) beacon frame[S214], the STA1 220 is unable to acquire the corresponding TIM element.

When the AP 210 transmits the beacon frame for the 6^(th) time [S216],the STA1 220 operates by switching to the awake state and is able toacquire the TIM element contained in the beacon frame [S224]. Since theTIM element is the DTIM that indicates that a broadcast frame ispresent, the STA1 220 does not transmit a PS-Poll frame to the AP 210but is able to receive a broadcast frame transmitted by the AP 210[S234]. Meanwhile, a wakeup interval set for the STA2 230 can be set tohave a period longer than that of the STA1 220. Hence, the STA2 230 canreceive the TIM element by switching to the awake state at the timingpoint 5215 at which the AP 210 transmits the beacon frame for the 5^(th)time [S241]. The STA2 230 recognizes that a frame to be transmitted tothe STA2 230 is present from the TIM element and is then able totransmit a PS-Poll frame to the AP 210 to request a frame transmission[S241 a]. Finally, the AP 210 is able to transmit a frame to the STA2230 in response to the PS-Poll frame [S233].

For the power save mode management shown in FIG. 16, TIM elementcontains TIM indicating whether a frame to be transmitted to STA ispresent or DTIM indicating whether a broadcast/multicast frame ispresent. And, the DTIM can be implemented through a field setup of theTIM element.

FIGS. 17 to 19 are diagrams to describe operations of an STA havingreceived TIM in detail.

Referring to FIG. 17, an STA switches to an awake state from a sleepstate in order to receive a beacon frame containing a TIM from an AP andis then able to recognize that there is a buffered traffic to betransmitted to the STA by interpreting the received TIM element. The STAperforms contention with other STAs for a medium access for a PS-Pollframe transmission and is then able to transmit a PS-Poll frame to makea request for a data frame transmission to the AP. Having received thePS-Poll frame transmitted by the STA, the AP is able to transmit a frameto the STA. The STA receives a data frame and is then able to transmitan ACK frame to the AP in response to the received data frame.Thereafter, the STA can switch to the sleep state again.

Like the example shown in FIG. 17, an AP can operate by an immediateresponse scheme in a manner of receiving a PS-Poll frame from an STA andthen transmitting a data frame after a lapse of a prescribed time (e.g.,SIFS (short inter-frame space). Meanwhile, after the AP has received thePS-Poll frame, if the AP fails to prepare the data frame, which is to betransmitted to the STA, within the SIFS time, the AP is able to operateby a deferred response scheme. This is described with reference to FIG.22 as follows.

In an example shown in FIG. 18, like the former example shown in FIG.21, an STA operates in a manner of switching to an awake state from asleep state, receiving a TIM from an AP, and then transmitting a PS-Pollframe to the AP. If the AP fails to prepare a data frame during SIFSdespite receiving the PS-Poll frame, the AP is able to transmit an ACKframe to the STA instead of transmitting the data frame. If the APprepares the data frame after transmitting the ACK frame, the APperforms a contending and is then able to transmit the data frame to theSTA. Subsequently, the STA transmits an ACK frame, which indicates thatthe data frame is successfully received, to the AP and is then able toswitch to the sleep sate.

FIG. 19 shows one example that an AP transmits a DTIM. Each of STAs canswitch to an awake state from a sleep state in order to receive a beaconframe containing a DTIM element from an AP. Each of the STAs can beaware that a multicast/broadcast frame will be transmitted through thereceived DTIM. After the AP has transmitted the beacon frame containingthe DTIM, the AP is able to immediately transmit data (i.e.,multicast/broadcast frame) without a PS-Poll frame transceivingoperation. Each of the STAs receives the data in the course of keepingthe awake state after receiving the beacon frame containing the DTIM andis then able to switch to the sleep state again after completion of thedata reception.

In a power save mode managing method based on TIM (or DTIM) protocoldescribed with reference to one of FIGS. 17 to 19, each of STAs cancheck whether a data frame, which will be transmitted for thecorresponding STA, is present through STA identification informationcontained in TIM element. The STA identification information may includean information related to an AID (association identifier) assigned tothe STA in the course of association with an AP.

The AID is used as a unique identifier for each STA in a single BSS. Forinstance, in a current WLAN system, the AID can be assigned as one ofvalues ranging 1 to 2,007. In a currently defined WLAN system, 14 bitscan be assigned to AID in a frame transmitted by an AP and/or STA and anAID value can be set to a value up to 16,383. Yet, 2,008 to 16,383 areset as reserved values.

FIG. 20 is a diagram for one example of TIM element format.

Referring to FIG. 20, a TIM element includes Element ID field, Lengthfield, DTIM Count field, DTIM period field, Bitmap Control field, andPartial Virtual Bitmap field. The length field indicates a length of aninformation field. The DTIM count field indicates how many beacon framesexist until a next DTIM is transmitted. The DTIM period field indicatesthe number of beacon spaces between contiguous DTIMs. If all TIM isDTIM, the DTIM period field has a value set to 1. The DTIM period fieldis reserved as 0 and consists of 1 octet. The bitmap control fieldconsists of a single octet. Bit 0 of the bitmap control field is atraffic indicator bit for AID 0. If at least one or more group addressedMSDUs/MMPDUs (MAC service data units)/MAC management protocol dataunits) have data to be sent by an AP or a mesh STA, the DTIM count fieldis set to 0 and the bit 0 of the bitmap control field is set to 1. Therest 7 bits in a 1^(St) octet indicate a bitmap offset. Atraffic-indication virtual bitmap by an AP or mesh STA for generatingTIM consists of 2,008 bits (=251 octets). The bit number N (0<=N<=2,007)in a bitmap can be expressed as the octet number N/8 and the bit number(N mod 8). Each bit in the traffic-indication virtual bitmap indicates apresence or non-presence of data to be sent by an AP. If data to be setby an AP for the individually addressed MSDU/MMPDU (AID=N) is present,the bit number N is set to 1. If not present, the bit number N is set to0.

The respective fields mentioned in the foregoing description correspondto examples of the fields that can be included in the TIM element, maybe substituted with other fields, or may further include additionalfields.

Power Management Using Automatic Power Saving Delivery

Aside from the above-described PS-Poll based power saving method, IEEE802.11e system provides an automatic power saving delivery (APSD)method. The ASPD is mainly categorized into a scheduled-APSD (s-APSD)method and an unscheduled-APSD (u-APSD) method. The u-APSD means amechanism for an AP (e.g., QoS AP) supportive of APSD to operate in apower saving mode having an awake state and a doze state switched toeach other and to deliver a downlink frame to an STA (e.g., QoS STA)supportive of APSD at the same time.

A QoS (quality of service) AP supportive of APSD can signal suchcapability to an STA using a beacon, a probe response and an APSDsubfield of a capability information field in an association(re-association) response management frame.

STAs can use u-APSD in order to receive bufferable units (BUs) of thecorresponding STAs, which are delivered from an AP in anunscheduled-service period (hereinafter abbreviated u-SP), entirely orin part. When the u-SP is not in progress, the u-SP can be initiated ifan STA transmits a QoS data or a QoS null frame belonging to an accesscategory (AC) set to ‘trigger-enabled’ to an AP. In this case, atransmitted uplink frame is named a trigger frame. Aggregated MPDU(A-MPDU) includes one or more trigger frames. The unscheduled-SP isterminated after the AP attempts a transmission of at least onescheduled BU for a delivery-enabled AC and the corresponding STA. Yet,if a maximum service period length field (Max SP Length field) of a QoScapability element of an association (re-association) request frame ofthe corresponding STA has a non-zero value, it is limited to a valueindicated in the corresponding field.

In order to receive BU from an AP in u-SP, an STA designates one or moreof a delivery-enabled AC and a trigger-enabled AC of the correspondingSTA. In IEEE 802.11e system, in order to provide QoS, 8 prioritiesdifferent from one another and 4 access categories (ACs) based on the 8different priorities are defined. An STA can configure an AP to useu-APSD using two kinds of methods. First of all, an STA can configure anindividual u-APSD flag bit in a QoS information (QoS Info) subfield of aQoS capability element delivered in an association (re-association)request frame. If the u-APSD flag bit is 1, it indicates that acorresponding AC is delivery-enabled and trigger-enabled. If all of 4u-APSD flag subfields in the association (re-association) request frameare set to 1, all ACs related to an STA are delivery-enabled andtrigger-enabled during the association (re-association). If all of 4u-APSD flag subfields in the association (re-association) request frameare set to 0, any delivery-enabled and trigger-enabled AC duringassociation (re-association) does not exist among the ACs related to theSTA. Alternatively, the STA is able to designate one or moredelivery-enabled and trigger-enabled ACs by transmitting a schedulesubfield set to 0 within a traffic stream (TS) information (Info) fieldof TSPEC (traffic specification) element in ADDTS (add traffic stream)request frame having APSD subfield set to 1 per AC to the AP. APSDconfiguration in TSPEC request may be prioritized over static u-APSDconfiguration delivered within the QoS capability element. In otherwords, the TSPEC request can be overwritten on the u-APSD configurationof any previous AC. And, the corresponding request can be transmittedfor an AC having an ACM subfield set to 0.

An STA is able to set an AC to be trigger-enabled or delivery-enabled ina manner of configuring TSPEC having an APSD subfield set to 1 and aschedule subfield set to 0 in an uplink or downlink transmissiondirection. Each of an uplink TSPEC, a downlink TSPEC and abi-directional TSPEC, each of which has an APSD subfield set to 1 and aschedule subfield set to 0, can configure an AC to be delivery-enabledand trigger-enabled. Each of an uplink TSPEC, a downlink TSPEC and abi-directional TSPEC, each of which has an APSD subfield set to 0 and aschedule subfield set to 0, can configure an AC to be delivery-disabledand trigger-disabled.

A scheduled-service period (hereinafter abbreviated s-SP) starts with afixed time interval specified in a service interval field. If an accesspolicy controls a channel access, in order to use s-SP for TS, an STAcan transmit an ADDTS request frame, which has an APSD subfield set to 1in a TS information field within a TSPEC element, to an AP. On the otherhand, if the access policy supports a contention-based channel access,in order to use s-SP for TS, an STA can transmit an ADDTS request frame,which has an APSD subfield set to 1 and a schedule subfield set to 1 ina TS information field within a TSPEC element, to an AP. If APSDmechanism is supported by the AP and the AP accepts the correspondingADDTS request frame from the STA, the AP can make a response with anADDTS response frame containing a schedule element indicating that arequested service can be provided by the AP. If 4 lower-ordered octetsof a TSF (timing synchronization function) timer are equal to aspecified value in a service start time field, an initial s-SP starts.An STA, which uses s-SP, can initially wake up in order to receive abuffered and/or polled BU individually addressed to itself from an AP ora hybrid coordinator (HC). Thereafter, the STA is able to wake up in apredetermined time interval equal to a service interval (SI). The AP isable to adjust a service start time through the schedule elements in asuccessful ADDTS response frame (i.e., a response to the ADDTS requestframe) and a schedule frame (transmitted at a different timing point).

The s-SP starts on the scheduled wake-up time corresponding to theservice start time and SI indicated within the schedule elementstransmitted in response to the TSPEC. Thereafter, the STA wakes up atthe timing point by Formula 9.

(TSF−service start time) mod minimum SI=0  [Formula 9]

If s-SP is supported in a BSS, an STA is able to use both u-APSD ands-APSD for different ACs on the same time. When an STA configures ascheduled delivery for an AC, an AP does not transmit BU, which uses thecorresponding AC, during an SP initiated by a trigger frame and does notprocess AC-using BU received from the STA in a trigger frame. The APdoes not decline any ADDTS request frame indicating to use both s-APSDand u-APSD to be used for the same AC on the same time. APSD can be usedfor a delivery of an individually addresses BU only. A group-addressedBU delivery may follow a frame delivery rule for a group-addressed BU.

A non-AP STA, which uses u-APSD, may not be able to receive all framestransmitted from an AP during a service period due to interferenceobserved by the corresponding non-AP STA. in this case, even if the APdoes not observe the same interference, the AP may be able to determinethat a frame is not accurately received by the non-AP STA. The u-APSDcoexistence capability can instruct a transmission duration requested tobe used for u-SP by the non-AP STA to the AP. Using the transmissionduration, the AP can transmit a frame during the SP and the non-AP STAcan improve reception possibility of a frame in an interferingsituation. The u-APSD coexistence capability lowers possibility inreceiving a frame during a service period by the AP.

FIG. 21 shows one example of U-APSD coexistence element format.

Referring to FIG. 21, an Element ID field is equal to a U-APSDcoexistence value. A length of additional subelements existing on 12 isadded to a value of a length field. A non-zero value in a TSF 0 Offsetfield means the number of microseconds after a time (TSF time 0) atwhich a non-AP STA recognizes that interference starts. An AP uses theTSF 0 Offset field for a transmission to the non-AP STA together with anInterval/Duration field.

An STA of which “dot11MgmtOptionUAPSDCoexistenceActivated” has a valueof ‘true’ is defined as an STA supportive of U-APSD coexistence. In thiscase, the STA of which “dot11MgmtOptionUAPSDCoexistenceActivated” has avalue of ‘true’ sets APSD Coexistence field of Extended Capabilitieselement to 1. Otherwise, the corresponding STA sets the field to 0. Anon-AP STA associated with an AP (if supporting U-APSD coexistencecapability was previously announced to both) is able to transmit ADDTSRequest frame containing U-APSD Coexistence element to the correspondingAP.

A content of ADDTS Request frame not containing U-APSD Coexistenceelement shall be named Base ADDTS Request hereinafter. If successfullyreceiving ADDTS Request frame, an AP processes the content of the BaseADDTS Request frame. If the AP determines that the Base ADDTS Request isnot acceptable, the AP does not process the U-APSD Coexistence element.On the contrary, if the AP determines that the Base ADDTS Request isacceptable, the AP processes the U-APSD Coexistence element. If the APsupports a frame transmission for a U-APSD service period for aspecified duration in Interval/Duration field of the U-APSD Coexistenceelement, the AP can grant the ADDTS request. Otherwise, the AP candecline the ADDTS request.

If the AP grants the ADDTS request previously having the U-APSDcoexistence, a non-AP STA, which keeps using a QoS service provided bythe ADDTS Request frame, is able to terminate the use of the U-APSDcoexistence by transmitting ADDTS Request frame not containing theU-APSD Coexistence element. If the non-AP STA desires to terminate theuse of all QoS services by the ADDTS Request frame containing the U-APSDcoexistence, the non-AP STA can transmit DELTS (delete traffic stream)Request frame to the AP.

If the previous ADDTS Request frame is nullifies by an ADDTS Requestframe successfully received last, the non-AP STA can transmit multipleADDTS Request frames to the AP. The AP, which supports the U-APSDcoexistence and accepts the ADDTS request, can restrict a U-APSD serviceperiod in accordance with a parameter specified in U-APSD Coexistenceelement of ADDTS frame. Moreover, the AP transmits a frame to make arequest to the non-AP STA by the following rules.

First of all, if the non-AP STA specifies a TSF 0 Offset value in theU-APSD Coexistence element into a non-zero value, the AP does nottransmit a frame to the non-AP STA out of the U-APSD coexistence serviceperiod. The U-APSD coexistence service time starts when the AP receivesa U-APSD trigger frame. Thereafter, the U-APSD coexistence service timeends after a transmission period specified by Formula 10.

End of transmission period=T+(Interval−((T−TSF 0 Offset) modInterval))  [Formula 10]

In Formula 10, T indicates a time at which a U-APSD trigger frame isreceived by an AP. And, Interval indicates an early arriving valueselected from Duration/Interval field value of the U-APSD Coexistenceelement and a timing point of a successful transmission having EOSP (endof service period) bit set to 1.

On the contrary, if the non-AP STA specifies the TSF 0 Offset value inthe U-APSD Coexistence element into 0, the AP does not transmit a frameto the non-AP STA out of the U-APSD coexistence service period. TheU-APSD coexistence service time starts when the AP receives a U-APSDtrigger frame. Thereafter, the U-APSD coexistence service time endsafter a transmission period specified by Formula 11.

End of transmission period=T+Duration  [Formula 11]

In Formula 11, T indicates a time at which a U-APSD trigger frame isreceived by an AP. And, Duration indicates an early arriving valueselected from Duration/Interval field value of the U-APSD Coexistenceelement and a timing point of a successful transmission having EOSP bitset to 1.

During the U-APSD coexistence service time, the AP further retains aframe which is to be transmitted by the corresponding AP. If it isdetermined that the corresponding frame will be successfully transmittedbefore expiration of the service period, an additional (more) bit can beset to 1.

If the AP estimates that a frame is a last frame that will betransmitted to the non-AP STA in the U-APSD coexistence service period,the AP can set the EOSP bit to 1 in the corresponding frame. If thecorresponding last frame is not successfully transmitted to the non-APSTA before an end of the U-APSD coexistence service period, the APtransmits a QoS null frame having the EOSP bit set to 1. The non-AP STAcan enter a doze state at the end timing point of the U-APSD coexistenceservice period.

Problem of Node Hidden in PS-Poll

FIG. 22 is a diagram to describe operations of STA according to PS-Pollmechanism and U-APSD mechanism.

FIG. 22 (a) shows one example of PS-Poll mechanism. And, FIG. 22 (b)shows one example of U-APSD mechanism.

Referring to FIG. 22 (a), an STA can be aware of a presence ornon-presence of a buffered traffic desired to be sent to thecorresponding STA by an AP through a TIM element of a beacon. If thetraffic to be transmitted to the STA is present, the STA performs acontending with other STAs by the PS-Poll mechanism and then makes arequest for a data frame transmission to the AP by transmitting aPS-Poll frame to the AP. After the AP has received the PS-Poll frame, ifthe AP fails to prepare a data frame to transmit to the STA within anSIFS time, the AP is able to transmit an ACK frame to the STA instead oftransmitting the data frame. Thereafter, if the AP prepares the dataframe after the ACK frame transmission, the AP performs a contendingwith other STAs, exchanges RTS/CTS frame with the STA, and thentransmits the data frame to the STA. In this case, the step ofexchanging the RTS/CTS frame can be omitted. If the STA successfullyreceives the data frame, the STA transmits an ACK frame to the AP andthen switches to a sleep state. Yet, in case of performing the datatransmission by the above-mentioned PS-Poll mechanism, since the AP cantransmit a single PSDU only at a time, if a size of data to be sent tothe STA by the AP is large, it is disadvantageous in that thetransmission may be performed inefficiently.

In order to settle such disadvantage, the STA can receive several PSDUsat a time from the AP using its own service period (SP) by theabove-mentioned U-APSD mechanism.

Referring to FIG. 22 (b), an STA recognizes that there is data desiredto be sent to the corresponding STA through a TIM element of a beacon.Thereafter, when the STA desires to receive the corresponding data, theSTA performs a contending with other STAs and then transmits a triggerframe to the AP in order to announce that a service period (SP) of theSTA has started and to make a request for the AP to transmit the data.Subsequently, the AP transmits an ACK frame to the STA in response tothe trigger frame. Thereafter, the AP performs a contending with otherSTAs, exchanges RTS/CTS frame with the STA, and then transmits the datato the STA. In doing so, the data can be configured with several dataframes. In this case, the step of exchanging the RTS/CTS frame can beomitted. When the AP transmits a last data frame, if the AP transmitsthe last data frame by setting an EOSP field of the corresponding dataframe to 1, the STA recognizes it, transmits an AC frame to the AP, andis then able to switch to a sleep state by ending the SP. Thus, if usingthe U-APSD mechanism, the STA can receive data by starting its own SP ona time desired by the STA and is able to receive several data frames ina single SP, whereby data reception can be efficiently performed.

Yet, the RTS/CTS frame exchange, which is required for the datatransmission to prevent the hidden node problem in the former example,causes a considerable amount of overhead to the data transmission.Moreover, after an STA has made a request for a transmission of data toan AP by transmitting a trigger frame, it takes a considerable time forthe AP to prepare data to send to the STA and to perform a contendingfor the transmission of the data. Hence, the STA consumes unnecessaryenergy.

Meanwhile, in a hidden node environment, there are user equipmentsunable to overhear PS-Poll frame transmitted by a different userequipment. And, it is highly probable that collision may occur due tosimultaneous transmissions of PS-Poll frames. To solve such problem, inorder for a PS mode user equipment to receive data from an AP in ahidden node environment, an NDP (null data packet) PS-Poll frame and anextended slot time based on the NDS PS-Poll frame can be used.

FIG. 23 is a diagram for one example of a case that PS-Poll framecollides in a hidden node environment.

In FIG. 23, assume that an AP retains data frames for an STA 1 and anSTA 2. And, assume that such a fact is announced to the STA 1 and theSTA 2 through a TIM element of a beacon frame. Moreover, assume that theSTA 1 and the STA 2 reciprocally correspond to hidden nodes.

Each of the STA 1 and the STA 2 attempts a channel access through ccontending. If backoff count values of the STA 1 and the STA 2 are 4 and6, respectively, as shown in FIG. 23, the STA 1 transmits a PS-Pollframe to the AP in the first place. If the PS-Poll frame of the STA 1 issuccessfully delivered to the AP, the AP transmits a buffered data framefor the STA 1 or an ACK frame to the STA 1. Yet, since the STA 2 is ahidden node of the STA 1, the STA 2 fails to monitor the PS-Poll frametransmitted by the STA 1 and then determines that a channel is idle on atime of transmitting the PS-Poll frame of the STA 1. Hence, the STA 2can perform a countdown of its backoff slot. Finally, if a countdownvalue of the backoff slot of the STA 2 expires, the STA 2 can transmit aPS-Poll frame to the AP as well. In particular, although the STA 1 hastransmitted the PS-Poll frame owing to the successful channel access inthe first place, the STA 2 transmits the PS-Poll frame due to the hiddennode problem. Hence, it may result in collision of PS-Poll frames.

In order to solve such a problem, it is necessary for a slot time of abackoff timer used for a contention process to be greater than a PS-Pollframe transmission time. In this case, the slot time corresponds to achannel idle time unit required for decreasing the backoff timer in thecontention process. Hence, if the slot time is set greater than thePS-Poll frame transmission time, an AP successfully receives a PS-Pollframe and is then able to transmit a response frame in response to thereceived PS-Poll frame. Since STAs corresponding to hidden nodes canreceive the response frame sent by the AP, they are aware that a channelis in use so as not to decrease the backoff timers. This considers aproblem that STAs located in a hidden node environment are unable tooverhear a PS-Poll frame. Hence, it is able to solve the problem of thehidden node by setting the slot time, i.e., a channel sensing time to begreater than the PS-Poll frame transmission time.

FIG. 24 is a diagram for one example of PS-Poll contention mechanism ina hidden node environment.

In FIG. 24, assume that an AP retains data frames for an STA 1 and anSTA 2. And, assume that such a fact is announced to the STA 1 and theSTA 2 through a TIM element of a beacon frame. Moreover, assume that theSTA 1 and the STA 2 reciprocally correspond to hidden nodes.

Each of the STA 1 and the STA 2 attempts a channel access through ccontending. If backoff count values of the STA 1 and the STA 2 are 4 and6, respectively, as shown in FIG. 23, the STA 1 transmits a PS-Pollframe to the AP in the first place. If backoff count values of the STA 1and the STA 2 are 1 and 2, respectively, the STA 1 transmits a PS-Pollframe to the AP in the first place. If the PS-Poll frame of the STA 1 issuccessfully delivered to the AP, the AP transmits a buffered data framefor the STA 1 or an ACK frame to the STA 1. Yet, since the STA 2 is ahidden node of the STA 1, the STA 2 fails to monitor the PS-Poll frametransmitted by the STA 1. The STA 2 determines that a channel is idle ona time of transmitting the PS-Poll frame of the STA 1 but alsodetermines that a channel is busy for a buffered data frame or an ACKframe transmitted after the PS-Poll frame. Therefore, the STA 2 does notperform a countdown of its backoff slot while the STA 1 occupies thechannel, whereby a situation of collision between PS-Poll frames can beavoided.

For the above-mentioned PS-Poll contention mechanism, a slot time can beset as Formula 12 in the following.

Slot Time=PS-Poll Transmission Time+SIFS+CCA Time of Responseframe+2*Air Propagation Delay  [Formula 12]

The PS-Poll Transmission Time means a transmission time of a PS-Pollframe. When an AP transmits a response frame (e.g., a data frame, an ACKframe, etc.) in response to a PS-Poll frame, the CCA Time of Responseframe means a CCA detection time of an STA.

According to the PS-Poll contention mechanism, in a hidden nodeenvironment, it is able to solve the PS-Poll collision problem. Yet, itcauses a problem that a time consumed in a contention process increasesdue to an increase of the slot time. As a method of mitigating such aproblem, it is able use an NDP PS-Poll frame shown in FIG. 25. Unlike anexisting PS-Poll frame is a MAC Control frame carried on PSDU, the NDPPS-Poll frame consists of PSDU-free NDP (Null Data Packet) only.

FIG. 25 is a diagram for one example of NDP PS-Poll frame.

Referring to FIG. 25, an NDP PS-Poll frame consists of STF, LTF and SIGfields only. Each of the STF and LTF fields consists of a channelestimation sequence necessary for decoding the SIG field. The SIG fieldcan mainly consist of 4 kinds of subfields. The fields exemplarily shownin FIG. 25 are just one example of subfields included in the SIG fieldof the NDP PS-Poll format, may be substituted with other fields, or mayhave other subfields added thereto. And, sizes of the respectivesubfields may have different values.

Type subfield is provided for SIG interpretation of an NPD frame andindicates that the corresponding NDP frame is designed for a PS-Pollframe. AID sub-field corresponds to an AID of an STA that transmits anNPD PS-Poll frame. The field is provided to enable an AP having receivedthe NDP PS-Poll frame to know which STA has transmitted the PS-Pollframe. Partial BSSID subfield corresponds to a portion of BSSID of an APto which the NDP PS-Poll frame transmitting STA belongs. Moreover,unlike this, any ID value for identifying the corresponding AP can beused. In particular, this may be used in a manner of defining a specificID for the AP or hashing the NSSID. For the usage of error detection ofthe SIG field, CRC subfield is included.

In case that an AP receives an NDP PS-Poll frame, the AP preferentiallydetermines whether to make a response to an STA in response to thePS-Poll frame through a partial BSSID subfield. The AP transmits an ACKframe to the corresponding STA in response to the NDP PS-Poll frame ormay transmit a buffered frame toward the corresponding STA.

In doing so, if the ACK frame is transmitted, it corresponds to a casethat the buffered frame toward the corresponding STA is not currentlypresent at the AP or a case that it is difficult to directly transmitthe buffered frame to the corresponding STA after SIFS. In case that thebuffered frame toward the corresponding STA is not present at the AP, amore data bit subfield in a frame control field of the ACK frame is setto 0. Otherwise, the more data bit subfield in the frame control fieldof the ACK frame is set to 1.

As mentioned in the foregoing description, it is able to solve thePS-Poll collision due to the existing hidden nodes using the NDP PS-Pollframe and the new extended slot time [cf. Formula 2]. Yet, by using theexisting contention-based PS-Poll mechanism, each user equipmentperforming PS-Poll receives a beacon and then recognizes whether adifferent user equipment uses a channel by continuously performing CCAuntil its PS-Poll is correctly transmitted. This causes unnecessarypower consumption to the user equipment on performing PS-Poll.Particularly, a power consumption of a user equipment, which performs alatest PS-Poll, will become relatively greater than that of another userequipment.

FIG. 26 is a diagram for one example of a channel access operation ofSTA using an extended slot time.

In FIG. 26, assume that an AP retains data frames for an STA 1, an STA 2and an STA 3. And, assume that such a fact is announced to the STA 1,the STA 2 and the STA 3 through a TIM element of a beacon frame.

Referring to FIG. 26, each of the STA 1, the STA 2 and the STA 3attempts a channel access through contention and performs a randombackoff using a new extended slot time. In the example shown in FIG. 26,the PS-Poll transmitting STAs select different backoff count values(e.g., STA 1=1, STA 2=2, STA 3=3), respectively.

In the following description, assume that an STA senses a busy (or,occupied) status of a medium for AIFS (arbitration interframe space)before PS-Poll frame transmission.

The STA 1 confirms that a medium is in an idle status during AIFS,counts down a backoff slot (i.e., 1 slot), and then transmits a PS-Pollframe to an AP. In doing so, each of the STA 2 and the STA 3 monitorsthat the medium is in a busy status and then stands by. The AP receivesthe PS-Poll frame from the STA 1 and then transmits a data frameimmediately after the SIFS. Subsequently, the STA 1 transmits an ACKframe in response to the data frame. Thus, while the STA 1 occupies themedium, each of the STA 2 and the STA 3 stops the countdown and standsby.

If the medium occupation by the STA 1 is ended, each of the STA 2 andthe STA 3 confirms that the medium is in the idle status during AIFS andthen performs a countdown of a residual backoff slot. Since a backoffcount value of the STA 2 is smaller than that of the STA 3, the STA 2counts down a residual backoff slot (i.e., 1 slot) and then transmits aPS-Poll frame to the AP. In doing so, the STA 3 monitors that the mediumis in a busy status and stands by. After the AP has received the PS-Pollframe from the STA 2, if the AP is unable to immediately transmit a dataframe after SIFS, the AP transmits an ACK frame after the SIFS. Thus,while the STA 2 occupies the medium, the STA 3 holds the countdown inthe backoff slot and stands by.

Once the medium occupation by the STA 2 is ended, the STA 3 confirmsthat the medium is in the idle status during the AIDS, performs acountdown of a residual backoff slot (i.e., 1 slot), and then transmitsa PS-Poll frame to the AP. After the AP has received the PS-Poll framefrom the STA 3, if the AP is unable to immediately transmit a data frameafter SIFS, the AP transmits an ACK frame.

Meanwhile, the AP performs a contending with STAs (e.g., performing anidle status check of a medium during AIFS and a random backoff) andtransmits data to the STA 2. Subsequently, the STA 2 transmits an ACKframe in response.

In the above-mentioned examples, since the STA 1, the STA 2 and the STA3 select the different backoff count values, respectively, collisiondoes not occur. Yet, since the rest of the STAs except the STA 1 deferPS-Poll during channel access periods of other STAs and keep the awakestate until receiving data toward themselves, unnecessary powerconsumption occurs. For instance, in case of the STA 2, aftertransmitting the PS-Poll, in order to receive data toward the STA 2, theSTA 2 keeps the awake status unnecessarily during the period for the STA1 to occupy the medium and the period for the STA 3 to occupy themedium. Moreover, this corresponds to a case that the slot time isextended greater than the PS-Poll transmission time. Hence, theunnecessary power consumption may increase greater than the existingpower consumption.

FIG. 27 is another diagram for one example of a channel access operationof STA using an extended slot time.

In FIG. 27, assume that an AP retains data frames for an STA 1, an STA 2and an STA 3. And, assume that such a fact is announced to the STA 1,the STA 2 and the STA 3 through a TIM element of a beacon frame.

Referring to FIG. 27, each of the STA 1, the STA 2 and the STA 3attempts a channel access through contention and performs a randombackoff using a new extended slot time. In the example shown in FIG. 27,each of the STA 2 and the STA 3 selects the same backoff count value(e.g., STA 1=1, STA 2=2, STA 3=2), respectively.

As mentioned in the foregoing description with reference to FIG. 26, ifthe medium occupation by the STA 1 is ended, each of the STA 2 and theSTA 3 confirms that the medium is in the idle status during AIDS andthen performs a countdown of a residual backoff slot. Yet, since abackoff count value of the STA 2 is equal to that of the STA 3, acollision occurs. Thus, in case that the collision occurs, since the STA2 and the STA 3 are unable to receive an ACK frame or a data frame fromthe AP, both of the STA 2 and the STA 3 fail in data transmission. Inthis case, each of the STA 2 and the STA 3 performs an exponentialbackoff. In particular, a CW value is doubled and a backoff count valueis reselected. In the example shown in FIG. 27, the STA 2 selects 5 asthe backoff count value and the STA 3 selects 7 as the backoff countvalue. Since the backoff count value of the STA 2 is smaller than thatof the STA 3, the STA 2 performs a countdown of backoff slots (i.e., 5slots) and then transmits a PS-Poll to the AP.

Thus, as backoff count values of two STAs coincide with each other, if acollision occurs, power consumptions of the two STAs increase and atransmission delay increases as well. Since this corresponds to a casethat a slot time is extended greater than a PS-Poll transmission time,unnecessary power consumption may become greater than the existing powerconsumption.

Improved Channel Access Method

In order to solve the above-mentioned problems, the present inventionproposes a method of reducing unnecessary power consumption inperforming PS-Poll after an STA recognizes that there is data to bedelivered to the STA by receiving a beacon containing a TIM. To thisend, in an improved channel access scheme, a channel access operationcan be performed in a channel access interval set per specific STA. Inthe following description of the present invention, a PS-Poll operation(i.e., an improved scheduled PS-Polling scheme) is assumed for a channelaccess, by which the present invention may be non-limited. And, a newframe for a channel access can be employed.

According to the present invention, when a random backoff of an STA isperformed, a new extended slot time mentioned in the foregoingdescription can be used, by which the present invention may benon-limited. And, an existing slot time can be used as well. In casethat an extended slot time is used, it can be determined as Formula 12.For PS-Poll, the aforementioned NDP PS-Poll frame having (STF+LTF+SIG)fields can be used, by which the present invention may be non-limited.And, an existing MAC control frame can be used.

An ACK frame may use an existing ACK frame identically or aconfiguration of an NDP ACK frame only having STF+LTF+SIG field similarto that of the aforementioned NDP PS-Poll frame. In this case, a size ofthe NDP PS-Poll frame may be equal to that of the NDP ACK frame.

In the following description, an STA can sense a busy (occupied) statusof a medium for one of SIFS, PIFS, (PIFS+additional time), and EDCA time(AIFS+Random backoff). In particular, after the STA has sensed a channelby selecting one of SIFS, PIFS, (PIFS+additional time), and EDCA time,if the channel is idle, the STA transmits a PS-Poll. In this case, theEDCA time indicates a channel sensing time previously used for anexisting STA to access a channel based on EDCA. For clarity of thefollowing description, assume that an STA uses PIFS.

FIG. 28 is a diagram for one example of a PS-Poll interval configuredper STA according to one embodiment of the present invention.

In FIG. 28, assume that an AP retains data frames for an STA 1, an STA 2and an STA 3. And, assume that such a fact is announced to the STA 1,the STA 2 and the STA 3 through a TIM element of a beacon frame.

Referring to FIG. 28, a PS-Poll interval of each of the STAs isdesignated per STA based on a TIM information element contained in abeacon and locations of the PS-Poll intervals are set different fromeach other among the STAs performing the PS-Poll. In particular, in casethat the AP stores data frames to transmit to the STAs, the AP canconfigure a PS-Poll interval for each of the corresponding STAs (STA 1,STA 2 and STA 3). In addition, in order to prevent other STAs (e.g.,STAs other than the STA 1, the STA 2 and the STA 3), which are notindicated by the TIM element, from attempting a channel access in totalPS-Poll interval, the AP can set a value of a duration field in a MACheader of the beacon in a manner of adding the total PS-Poll interval toa length of the corresponding beacon. Since each of the STAs can checkthe length of the beacon through a length field and an MCS field withinan SIG field, it can be aware of the total PS-Poll interval designatedto the STAs (the STA 1, the STA 2 and the STA 3) indicated by the TIMelement through the duration field. And, other STAs (STAs other than theSTA 1, the STA 2 and the STA 3) not indicated by the TIM element may notattempt a channel access in the corresponding total PS-Poll interval.

An AP is able to explicitly inform STAs of location information of aPS-Poll interval through an information element (e.g., a TIM informationelement, a polling assignment information element, etc.) of a beaconframe. In particular, the AP can additionally inform each STA, which isindicated by a TIM element, of the location information of the PS-Pollinterval through the information element in the beacon frame. Forinstance, the AP can inform each STA of an offset information on a starttiming point of the PS-Poll interval and a length information of thePS-Poll interval of the corresponding STA. In this case, the lengthinformation of the PS-Poll interval may differ per STA. If all STAs usethe same PS-Poll interval length, a single PS-Poll interval lengthinformation is contained in the information element within the beaconframe and each of the corresponding STAs acquires its PS-Poll intervalinformation using the corresponding PS-Poll interval length. If a lengthof a PS-Poll interval of each STA is fixed or the corresponding STA isable to know the length of the PS-Poll interval implicitly (e.g., if aPS-Poll interval length is determined by a system, PIFS+PS-Poll frametransmission time+SIFS+CCA Time of Response frame (e.g., ACK frametransmission time)+2*Air Propagation Delay), the AP can inform the STAof an information on a start timing point of total PS-Poll interval andan order information of a PS-Poll of each STA indicated by a TIM througha TIM element. In this case, each STA can obtain a location of itsPS-Poll interval by checking an order of its PS-Poll interval from thestart timing point of the total PS-Poll interval using the PS-Poll orderinformation. If a length of a PS-Poll interval of each STA is fixed orthe corresponding STA can be aware of the length of the PS-Poll intervalimplicitly, and if a start timing point of total PS-Poll interval isfixed (e.g., if the total PS-Poll interval starts directly from a timingpoint designated after a beacon reception), the AP can announce theorder information of the PS-Poll of each STA indicated by a TIM througha TIM element.

Alternatively, an STA can obtain a location information of its PS-Pollinterval implicitly through a TIM element. For instance, assuming thatSTA 1, STA 2 and STA 3 are sequentially indicated by a partial virtualbitmap field of the TIM element and that the PS-Poll order correspondsto an ascending order of a bitmap, the STA can have its own interval inorder of STA 1, STA 2 and STA 3. Thus, as mentioned in the abovedescription, the PS-Poll order of each STA can be previously determinedby a system in a manner of having an ascending or descending order inaccordance with the order of the bitmap. Alternatively, each STA may beable to calculate its own PS-Poll order using a bitmap order based on apredetermined specific permutation.

Thus, one STA is able to obtain a location of its PS-Poll interval and alocation of a PS-Poll interval of another STA based on informationcontained in a TIM. Having confirmed the location of its own PS-Pollinterval, the corresponding STA is able to perform a PS-Poll operation.Moreover, an STA checks whether a medium is busy during PIFS at a starttiming point of its own PS-Poll interval. If the medium is in idlestatus, the STA can transmit a PS-Poll to an AP. If a channel is busyduring the PIFS at the start timing point of the PS-Poll interval, theSTA can defer a PS-Poll frame transmission in its own PS-Poll interval.Thereafter, if the STA confirms that the medium is in idle status, theSTA can transmit the deferred PS-Poll frame to the AP. In doing so,since the rest of STAs are not in their own PS-Poll intervals, they canoperate in sleep state.

If the AP receives the PS-Poll frame from the corresponding STA in thePS-Poll interval, the AP transmits an ACK frame to the corresponding STAafter SIFS. If the AP fails in correctly receiving the PS-Poll framefrom the corresponding STA in the PS-Poll interval, the AP can transmita frame containing NACK or an ACK frame to the corresponding STA. Inthis case, the NAC or ACK frame can include an NDP frame.

For clarity of the following description, assume that an STA is able toimplicitly obtain a location of its own PS-Poll interval through a TIMelement.

If a 1^(st) PS-Poll interval in a corresponding beacon period startsdirectly after a beacon reception, a PS-Poll interval of one STA can bedetermined as Formula 13 in the following.

PS-Poll interval=SIFS(or PIFS)+PS-Poll transmission time+SIFS+ACKtransmission time+2*Air Propagation Delay  [Formula 13]

Unlike Formula 13, If a 1^(st) PS-Poll interval in a correspondingbeacon period starts directly after SIFS or PIFS after a beaconreception, a PS-Poll interval of one STA can be determined as Formula 14in the following.

PS-Poll interval=PS-Poll transmission time+SIFS+ACK transmissiontime+SIFS(or PIFS)+2*Air Propagation Delay  [Formula 14]

If an NDP PS-Poll frame and an NDP ACK frame are used, a PS-Pollinterval of one STA can be determined as Formula 15 in the following.

2*NDP frame transmission time+SIFS(or PIFS)+SIFS+2*Air PropagationDelay  [Formula 15]

Total PS-Poll interval can be determined as Formula 16 in the following.

(SIFS(or PIFS)+PS-Poll transmission time+SIFS+ACK time)*N+2*AirPropagation Delay*N,

or (2*NDP frame transmission time+SIFS(or PIFS)+SIFS)*N+2*AirPropagation Delay*N  [Formula 16]

In Formula 16, the N means the total number of STAs set to 1 in a TIMbitmap, i.e., STAs that will perform PS-Poll.

In the present specification, for clarity of the following description,a PS-Poll interval of one STA can be determined as Formula 13.

Thereafter, each of the STAs having performed the PS-Poll in theirPS-Poll interval switches to an awake state to receive data from an APafter the total PS-Poll interval.

FIG. 29 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In FIG. 29, assume that an AP retains data frames for an STA 1, an STA 2and an STA 3. And, assume that such a fact is announced to the STA 1,the STA 2 and the STA 3 through a TIM element of a beacon frame.

Referring to FIG. 29, if 3 STAs (STA 1, STA 2 and STA 3) aresequentially indicated by a TIM of a beacon, each of the STAs can have aPS-Poll interval configured in order of the STA 1, the STA 2 and the STA3. And, each of the STAs can obtain a location of its own PS-Pollinterval based on an information contained in the TIM.

First of all, a PS-Poll interval of the STA 1 starts after a receptionof a beacon. Having received the beacon, the STA 1 corresponding to a1^(st) STA confirms an idle status of a medium during PIFS and thentransmits a PS-Poll frame to an AP. Since each of the STAs is able toknow its own PS-Poll interval, the rest of the STAs except the STAperforming a 1^(St) PS-Poll switches to a sleep state after the beaconreception and are then able to maintain the sleep state until their ownPS-Poll intervals, respectively. In FIG. 29, each of the STA 2 and theSTA 3 switches to the sleep state after the beacon reception and thenmaintain the sleep state until its own PS-Poll interval. After receivingthe PS-Poll frame from the STA 1, the AP transmits an ACK frame afterSIFS. And, the PS-Poll interval for the STA 1 ends. The STA except theSTA (e.g., STA 3) performing a last PS-Poll switches to the sleep stateat an end timing point of its own PS-Poll interval and is then able tomaintain the sleep state until an end point of total PS-Poll interval.In FIG. 29, the STA 1 switches to the sleep state at an end timing pointof its own PS-Poll interval and then maintains the sleep state until theend timing point of the total PS-Poll interval.

A PS-Poll interval of the STA 2 starts from an interval right next tothe PS-Poll interval of the STA 1 (i.e., after a time of an ACK frametransmission to the STA 1). If the STA 2 switches to an awake state andthen confirms a busy status of a medium during PIFS, the STA 2 transmitsa PS-Poll frame to the AP. After receiving the PS-Poll frame from theSTA 2, the AP transmits an ACK frame after SIFS. And, the PS-Pollinterval for the STA 2 ends. Like the STA 1, the STA 2 switches to thesleep state at an end timing point of its own PS-Poll interval and thenmaintains the sleep state until the end timing point of the totalPS-Poll interval.

A PS-Poll interval of the STA 3 starts from an interval right next tothe PS-Poll interval of the STA 2 (i.e., after a time of an ACK frametransmission to the STA 2). If the STA 3 switches to an awake state andthen confirms a busy status of a medium during PIFS, the STA 3 transmitsa PS-Poll frame to the AP. After receiving the PS-Poll frame from theSTA 3, the AP transmits an ACK frame after SIFS. Since the STA 3 is alast STA (i.e., an timing point of its own PS-Poll interval is equal toan end timing point of the total PS-Poll interval), the STA 3 does notswitch to the sleep state but maintains the awake state.

After the end of the total PS-Poll interval, the AP transmits data toeach of the STAs. Each of the STAs (except the last STA) switches to theawake state at the end timing point of the total PS-Poll interval andthen performs a CCA to receive the data from the AP. The last STA (i.e.,the STA 3) maintains the awake state from its own PS-Poll interval andperforms the CCA. The AP is able to transmit the data to each of theSTAs by performing a contention based on a random backoff period. Inother words, the AP selects a backoff count value for each STA and isthen able to transmit data to an STA having a smallest backoff countvalue to an STA having a greatest backoff count value in order ofbackoff count value. In the example shown in FIG. 29, the STA 1 has asmallest backoff count value, the STA 3 has a greatest backoff countvalue, and the STA 2 has a backoff count value between the smallest andthe greatest. The AP confirms that a medium is in idle status duringAIFS, counts down a backoff slot, and then transmits a data frame to theSTA 1. The STA is able to check whether the data frame is a data frametoward the corresponding STA through a preamble (e.g., a partial AID ofan SIG field) of the data frame transmitted from the AP. In particular,the STA 1 confirms that the transmitted data frame is the data frametoward the STA 1 and decodes the corresponding data frame. In doing so,each of the rest of the STAs (i.e., STA 2, STA 3) confirms that thetransmitted data is not the data frame toward the corresponding STA andis then able to switch to a sleep mode. The STA can obtain a length ofan MPDU of the data frame transmitted from the AP through a preamble(e.g., a length of SIG field) of the corresponding data frame. Inparticular, each of the STAs, which switched to the sleep mode byconfirming that the transmitted data frame was not the data frame towardthe corresponding STA, can switch to the awake state by considering alength of an MPDU of other STA.

Having received the data frame from the AP, the STA 1 transmits an ACKframe to the AP after SIFS. At a timing point of transmitting the ACKframe to the AP from the STA 1, i.e., at an end timing point of a datatransmission from the AP to the STA 1, each of the STA 2 and the STA 3switches to the awake state from the sleep state and then receives adata frame from the AP on the basis of contention.

On the other hand, an AP may be able to transmit data frames to STAs onthe basis of non-contention. For instance, in a PS-Poll interval of eachSTA, the AP can transmit a scheduling information to the correspondingSTA. In this case, after the STA has received an ACK frame for a PS-Pollframe from the AP in its own PS-Poll interval, the STA can wait for ascheduling information on a downlink data transmission from the AP afteran end of total PS-Poll interval. The STA is then able to receive datausing the obtained scheduling information. Thus, when the STA obtainsthe scheduling information from the AP, the STA is able to minimizepower consumption by switching to a sleep state until a downlink datatransmission timing point.

Moreover, the AP is able to directly transmit data to a last STA, whichperforms a PS-Poll, without an ACK frame transmission for the PS-Poll,which is described with reference to FIG. 30 as follows.

FIG. 30 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In the example shown in FIG. 30, an operation for each STA to transmit aPS-Poll frame to an AP in its own PS-Poll interval is identical to thatshown in FIG. 29.

Referring to FIG. 30, after an AP has received a PS-Poll frame from anSTA 3 corresponding to a last STA, if the AP confirms that the STA 3 isthe last STA, the AP does not transmit an ACK frame for the PS-Pollframe but transmits a data frame to the STA 3 after SIFS.

After total PS-Poll interval has ended, the AP can transmit data to therest of STAs except the last STA (i.e., STA 3). And, the rest of theSTAs except the last STA can switch to an awake state at an end timingpoint of the total PS-Poll interval. In this case, since each of the STA1 and the STA 2 switches to the awake state in the course oftransmitting the data frame to the STA 3 (i.e., at the end timing pointof the total PS-Poll interval), it may be unable to check a preamble ofthe data frame transmitted to the STA 3. Hence, each of the STA 1 andthe STA 2 maintains the awake state. Thereafter, after confirming thatthe corresponding frame is destined to the STA 1 through the preamble ofthe data frame transmitted from the AP, the STA 1 decodes thecorresponding data frame and the STA 2 can switch to the sleep stateagain.

FIG. 30 just shows one example that each of the STA 1 and the STA 2maintains the awake state by switching to the awake state at the endtiming point of the total PS-Poll interval until receiving the dataframe from the AP. Yet, after the rest of the STAs have switched to theawake state at the end timing point of the total PS-Poll interval, theyswitch to the sleep state if confirming a busy status of a medium.Subsequently, the rest of the STAs switch to the awake state again at anend timing point of the medium occupation and may be able to checkwhether data frames are transmitted to the corresponding STAs,respectively.

An AP can transmit data to the rest of STAs except a last STA byperforming a contention based on a random backoff period. An operationfor the AP to transmit the data to the rest of the STAs except the lastSTA is identical to that of the example shown in FIG. 29 and its detailsshall be omitted from the following description.

Moreover, an AP can transmit a data frame to an STA having a delaysensitive packet most preferentially after a lapse of total PS-Pollinterval. This is described in detail with reference to FIG. 31 asfollows.

FIG. 31 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In the example shown in FIG. 31, an operation for each STA to transmit aPS-Poll frame to an AP in its own PS-Poll interval is identical to thatshown in FIG. 29.

Referring to FIG. 31, after total PS-Poll interval, an AP stands by forPIFS (or SIFS) and is then able to transmit a data frame to adelay-sensitive data frame transmitted STA among polled STAs directlywithout a random backoff procedure. In particular, data can betransmitted without contention. This can prevent a delay-sensitivepacket transmission delay according to a random backoff based datatransmission operation with a long slot time. Of course, in this case, acontention-based data transmission can be performed on an STA not havinga delay sensitive packet like the former example shown in FIG. 29 orFIG. 30. In this case, an end timing point of the total PS-Poll intervalmay correspond to a timing point of transmitting a response to an STAthat has performed a last PS-Poll. For instance, in case of the formerexample shown in FIG. 29, the end timing point of the total PS-Pollinterval may mean a timing point of transmitting (NDP) ACK frame to theSTA having performed the last PS-Poll. For another instance, in case ofthe former example shown in FIG. 30, the end timing point of the totalPS-Poll interval may mean a timing point of transmitting a data frame tothe STA having performed the last PS-Poll. In FIG. 31, the end timingpoint of the total PS-Poll interval corresponds to a timing point oftransmitting (NDP) ACK frame to the STA 3 having performed the lastPS-Poll. And, FIG. 31 shows one example that after the total PS-Pollinterval, the AP transmits downlink data frames for the STA 1 and theSTA 3 right after PIFS. Since the STA 2 does not have a delay sensitivepacket, the data transmission to the STA 2 can be performed by acontention based mechanism (i.e. AIFS+Random backoff) like the formerexample shown in FIG. 29 or FIG. 30.

Meanwhile, it may happen that all STAs designated to TIM are unable toreceive the TIM. For instance, interference due to OBSS (overlappingBSS) transmission may happen or an STA may miss the TIM. In this case,the STA failing to receive the TIM is unable to transmit a PS-Poll framein its own PS-Poll interval. And, it may happen that the AP fails inreceiving a PS-Poll frame despite that the STA transmits the PS-Pollframe correctly. In such case, an operation between the AP and the STAis described in detail with reference to FIG. 32 as follows.

FIG. 32 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In the example shown in FIG. 32, an operation for each STA to transmit aPS-Poll frame to an AP in its own PS-Poll interval is identical to thatshown in FIG. 29.

Referring to FIG. 32, a case that an AP is unable to receive a PS-Pollframe from an STA 2 is illustrated. If the AP fails in receiving aPS-Poll from each STA on a PS-Poll transmission time in a determinedPS-Poll interval of the corresponding STA, the AP transmits anunsolicited NDP frame to the corresponding STA after SIFS. A timingpoint of transmitting the unsolicited NDP frame may be equal to a timingpoint (PIFS+PS-Poll Transmission Time+SIFS) of transmitting (NDP) ACKframe on the assumption of a case that the corresponding STA normallytransmits the PS-Poll. In this case, the unsolicited NDP frame may havea configuration of one of (NDP) ACK frame, new NDP frame and new MACframe used in FIGS. 29 to 31. And, the unsolicited NDP frame may containACK, NACK or non-polling (no polling) indication. Thus, although the APfails in receiving a PS-Poll frame at a determined timing point from adetermined STA, the AP can prevent intervention into a PS-Poll intervalby another STA (e.g., a hidden node) by transmitting NDP frame (or newframe).

Having received the NDP frame containing the ACK, NACK or non-polling(no polling) indication, the STA can perform a contention-based PS-Poll.In this case, the STA can use an extended slot time. While a medium isoccupied by each of the STAs (e.g., STA 1, STA 3) in a PS-Poll intervalassigned to the corresponding STA [not shown in FIG. 32, although threeis another STA failing to receive the TIM, since the AP transmits theNDP frame containing the NACK or non-polling (no polling) indication inthe PS-Poll interval of the corresponding STA, the STA having receivedthe NDP frame containing the NACK or non-polling (no polling) indicationis able to eventually perform the contention-based PS-Poll after thetotal PS-Poll interval.

Thus, in case that a slot time is determined as an extended slot timeaccording to Formula 12, like the example shown in FIG. 32, it is ableto prevent the intervention of another STA in the corresponding PS-Pollinterval by transmitting the NDP frame. Yet, in case that the slot timedoes not have the extended slot time, the example shown in FIG. 32 maynot apply thereto. For instance, when a slot time is equal to a PS-Pollframe time, if intervention of another STA occurs for a time of(PIFS+PS−Poll Transmission Time+SIFS), it may not be able to prevent theintervention of another STA through the example shown in FIG. 32. Yet,in this case, since an STA indicated through a TIM element of a beaconframe performs a PS-Poll using its own PS-Poll interval and STAs notindicated by the TIM element can obtain total PS-Poll interval from thebitmap number of the TIM element or a value of a duration field of a MACheader of the beacon, the AP does not attempt a channel access in thetotal PS-Poll interval, thereby preventing the above-mentioned problemfrom being caused.

Meanwhile, when an STA receives a TIM and attempts to perform a PS-Pollin a designated PS-Poll interval, a channel may be occupied (or busy).Such a status shall be described in detail with reference to FIG. 33 asfollows.

FIG. 33 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In the example shown in FIG. 33, an operation for each STA to transmit aPS-Poll frame to an AP in its own PS-Poll interval is identical to thatshown in FIG. 29.

Referring to FIG. 33, when an STA receives a TIM and attempts to performa PS-Poll in a PS-Poll interval designated to the corresponding STA, achannel may be in a busy status. In particular, FIG. 33 shows oneexample of a status that a channel is occupied by a transmission of OBSS(overlapping BSS) when a STA 2 attempts a PS-Poll. In this case, whenthe STA 2 attempts the PS-Poll in its own PS-Poll interval, if the STA 2determines that the channel is occupied (due to the OBSS transmission),the STA 2 defers the PS-Poll transmission until the channel enters anidle status. If the channel is idle during AIFS, the STA 2 attempts thePS-Poll transmission again. In particular, the corresponding STA doesnot transmit a PS-Poll frame in the PS-Poll interval designated to thecorresponding STA but attempts the PS-Poll transmission by an existingcontention-based (AIFS+Random backoff) using an extended slot time. Inparticular, in this case, as mentioned in the foregoing description withreference to FIG. 32, since a PS-Poll interval is designated to eachpolled STA in total PS-Poll interval, the corresponding STA can performthe PS-Poll on the basis of contention after the total PS-Poll interval.

Meanwhile, the AP may be able to transmit an STF instead of the (NDP)ACK frame used in the examples shown in FIGS. 29 to 33. As the STF isused instead of the (NDP) ACK frame, the (NDP) ACK frame can betransmitted after the total PS-Poll interval.

In this case, a PS-Poll interval of each STA can be determined asFormula 17 in the following.

PS-Poll interval=PIFS (or SIFS)+PS-Poll transmission+SIFS+STF+2*AirPropagation Delay  [Formula 17]

At a timing point (e.g., after (PIFS/SIFS+PS-Poll TransmissionTime+SIFS)) designated in a PS-Poll interval of each STA, the APtransmits the STF instead of the (NDP) ACK frame. The STF transmitted bythe AP performs a function of informing other STAs that a channel isoccupied by a designated STA in a designated PS-Poll interval, therebyprohibiting an intervention by a hidden node STA. Since a size of theSTF is smaller than that of an existing ACK frame or an NDP frame, asize of the total PS-Poll interval may decrease. Therefore, STA's powerconsumption can be reduced.

The total PS-Poll interval can be determined as Formula 18 in thefollowing.

Total PS Poll Interval=(PIFS (or SIFS)+PS-Poll transmissiontime+SIFS+STF time)*N+2*Air Propagation Delay*N  [Formula 18]

In Formula 18, the N means the total number of STAs set to 1 in a TIMbitmap, i.e., STAs that will perform the PS-Poll.

If a PS-Poll frame includes an NDP frame, the total PS-Poll interval canbe determined as Formula 19 in the following.

Total PS Poll Interval=(PIFS (or SIFS)+NDP frame transmissiontime+SIFS+STF time)*N+2*Air Propagation Delay*N  [Formula 19]

FIG. 34 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In the example shown in FIG. 34, an operation for each STA to transmit aPS-Poll frame to an AP in its own PS-Poll interval is identical to thatshown in FIG. 29 except that the AP transmits an STF instead of (NDP)ACK frame in response to the PS-Poll frame.

Referring to FIG. 34, after an end of a total PS-Poll interval, an APtransmits an ACK frame to all STAs. Each of the STAs (except a last STA)switches to an awake state at an end timing point of the total PS-Pollinterval and then performs a CCA to receive the ACK frame from the AP.The last STA (i.e., STA 3) maintains the awake state from its ownPS-Poll interval and performs the CCA. If the AP receives PS-Poll framesfrom all STAs indicated by TIM, the AP is able to transmit a single ACKframe by broadcast. In doing so, the ACK frame can be transmitted informat of an existing ACK frame or an NDP ACK frame. Yet, if the APfails in receiving the PS-Poll frames from all STAs indicated by theTIM, the AP can transmit an ACK frame per STA or can transmit a groupACK frame containing a bitmap information indicating an ACK per STA bybroadcast. Hereinafter, such a group ACK frame may be named a PS-PollGroup ACK (PPGA) frame.

After the AP has transmitted the ACK frame to all STAs, the AP transmitsdata to each of the STAs after the transmission of the ACK frame [notshown in FIG. 34]. Subsequently, each of the STAs performs CCA toreceive the data from the AP. In particular, the AP can transmit thedata to each STA by performing a contention based on a random backoffperiod. A process for each STA to receive data from an AP can beperformed in the same manner according to the example shown in FIG. 29.

In the following description, the NDP ACK frame and the PPGA frame areexplained in detail.

FIG. 35 is a diagram for one example of NDP ACK frame according to oneembodiment of the present invention.

Referring to FIG. 34 and FIG. 35, an NDK ACK frame may include STF, LTFand SIG field. The SIG field may include an NDP ACK indication subfield,an AID (or partial AID) subfield, a more data subfield and a CRCsubfield. In this case, each of the subfields corresponds to one exampleof subfield that can be included in the NDP ACK frame, may besubstituted with a different subfield, or may further include anadditional subfield.

The NDP ACK indication subfield indicates that a corresponding frame isan NDP ACK frame. Through the NDP ACK indication field, an STA can beinformed of an ACK for a PS-Poll. The AID (or partial AID) subfieldindicates an STA that receives a corresponding NDP ACK frame. The moredata subfield indicates whether a buffered frame destined to an STAreceiving a corresponding NDP ACK frame exists in an AP. And, the CRCsubfield is used for the usage of error detection for the SIG field.

Like the example shown in FIG. 34, if an AP receives PS-Poll frames fromall STAs indicated by a TIM, the AID (or partial AID) subfield can beset to a specific value (e.g., all bits set to 1 or 0) indicatingmulticast/broadcast in order to enable the STAs transmitting the PS-Pollto receive NDP ACK frame (i.e., in order to indicate that an ACK istransmitted by broadcast). After the STA has received the NDP ACK framecontaining the AID (or partial AID) subfield set to the specific value,if the STA is the STA having performed the PS-Poll, the correspondingSTA is able to check whether the received NDP ACK frame is a group ACKfor the PS-Poll. In particular, when the STA receives the NDP ACK frame,if the AID indicates all STAs having performed the PS-Poll, each of theSTAs having performed the PS-Poll determines that the received NDP ACKframe is the ACK transmitted to the corresponding STA and then reads theSIG field. On the contrary, each of the STAs not having performed thePS-Poll can ignore the corresponding NDP ACK frame.

On the other hand, unlike the example shown in FIG. 34, if the AP isunable to receive PS-Poll frames from all user equipments indicated by aTIM, the AP can transmit an NDP ACK frame for each STA havingtransmitted the PS-Poll frame. In this case, the AID (or partial AID)subfield can be set to an AID (or a partial ID) of the STA receiving thecorresponding NDP ACK frame. And, the AP may be able to transmit aPS-Poll group ACJ (PPGA) frame to all STAs as follows.

FIG. 36 is a diagram for one example of PS-Poll group ACK frameaccording to one embodiment of the present invention.

Referring to FIG. 34 and FIG. 36, a PPGA frame shown in FIG. 36 (a) mayinclude a frame control field, an AID field (or an RA (receiver address)field), a BSSID field, a bitmap size field, an ACK bitmap field, and apadding field. In case that the PPGA frame is configured by includingthe RA field, the RA field can have a size of 6 octets. In this case,each of the subfields corresponds to one example of subfield that can beincluded in the PPGA frame, may be substituted with a differentsubfield, or may further include an additional subfield.

A type subfield and a subtype subfield within the frame control fieldindicate that a corresponding frame is a group ACK. The AID fieldindicates an STA that receives a corresponding PPGA frame. In order toenable all STAs having performed PS-Poll to receive the PPGA frame, theAID (or RA) field can be set to a broadcast address (e.g., all bits setto 1 or 0). The bitmap size field indicates a size of the ACK bitmapfield and is set to the number set to 1 in the TIM (i.e., the number ofuser equipments having performed PS-Poll). Namely, only the STA, whichhas read the TIM correctly and performed the PS-Poll, is able to readthe ACK bitmap field. In case of the STA of which PS-Poll frame isreceived by the AP, the ACK bitmap field is set to 1. In case of the STAof which PS-Poll frame is not received by the AP, the ACK bitmap fieldis set to 0. In this case, the ACK bitmap field can be configured in thesame order of a bitmap of TIM element.

Referring to FIG. 36 (b), a PPGA frame does not include the bitmap sizefield shown in FIG. 36 (a) and may include an ACK bitmap only. In thiscase, each user equipment having performed PS-Poll is able to calculatea size of the ACK bitmap in the PPGA frame through TIM information. Forinstance, the size of the ACK bitmap within the PPGA frame can be equalto that of a bitmap within TIM element.

Referring to FIG. 36 (c), a PPGA frame can include a compressed MACheader (or a new MAC header). And, the PPGA frame can be configuredwithout AID (or RA field) shown in FIG. 36 (b). In this case, after aTIM has been received, each STA having performed a PS-Poll can recognizethat a corresponding frame is a group ACK through a type subfield and asubtype subfield within a frame control field.

An AP is able to directly transmit (NDP) ACK frame or PPGA frame withouttransmitting STF in response to a last PS-Poll, which is described indetail with reference to FIG. 37 as follows.

FIG. 37 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In the example shown in FIG. 37, an operation for each STA to transmit aPS-Poll frame to an AP in its own PS-Poll interval is identical to thatshown in FIG. 29 except that an AP transmits an STF instead of (NDP) ACKframe in response to the PS-Poll frame.

Referring to FIG. 37, after an AP has received a PS-Poll frame from anSTA 3 corresponding to a last STA, the AP transmits (NDP) ACK frame orPPGA frame instead of SIF right after SIFS. In this case, each STAhaving performed a PS-Poll (except the last STA) switches to an awakestate at a timing point of (Total PS-Poll Interval-STF-SIFS) and thenperforms a CCA to receive an ACK from the AP. The last STA (i.e., STA 3)performs a CCA by maintaining an awake state from its own PS-Pollinterval.

Meanwhile, as mentioned in the foregoing description, since all STAsdesignated to a TIM are unable to receive the TIM, it may happen thateach of the STAs is unable to transmit a PS-Poll frame in its ownPS-Poll interval. On the other hand, although an STA transmits a PS-Pollframe correctly, it may happen that an AP is unable to receive thePS-Poll frame. In such cases, like the former example that an APtransmits an unsolicited NDP frame, the AP can transmit an unsolicitedSTF, which is described in detail with reference to FIG. 38 as follows.

FIG. 38 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

In the example shown in FIG. 38, an operation for each STA to transmit aPS-Poll frame to an AP in its own PS-Poll interval is identical to thatshown in FIG. 29 except that the AP transmits an STF instead of (NDP)ACK frame in response to the PS-Poll frame.

Referring to FIG. 38, a case that an AP is unable to receive a PS-Pollframe from an STA 2 is illustrated. If the AP fails in receiving aPS-Poll from each STA at a PS-Poll transmission time in a PS-Pollinterval of the corresponding STA, the AP transmits an unsolicited STFto the corresponding STA after SIFS. A timing point of transmitting theunsolicited STF may be equal to a timing point (PIFS+PS-PollTransmission Time+SIFS) of transmitting an STF on the assumption of acase that the corresponding STA transmits a PS-Poll normally. Thus,although an AP fails in receiving a PS-Poll frame at a determined timingpoint from a determined STA, the AP transmits an unsolicited STFD toprevent intervention in a PS-Poll interval by another STA (e.g., ahidden node).

Meanwhile, an unsolicited NDP frame or STF can be used for an AP toprevent a channel access by a different STA in situations as well as ina situation of the aforementioned example, which is described in detailwith reference to FIG. 39 as follows.

FIG. 39 is a diagram to describe a channel access operation of an STAaccording to one embodiment of the present invention.

Referring to FIG. 39, once a random backoff process is initiated toaccess a channel, an STA 1 monitors a medium continuously while countingdown a backoff slot in accordance with a determined backoff count value.In order to prevent channel accesses by other STAs, an AP transmits anunsolicited NDP frame/STF in a specific slot. Since it is monitored thatthe medium is in a busy status in the slot in which the unsolicited NDPframe/STF is transmitted from the AP, the corresponding STA holds thecountdown and stands by. If the medium enters an idle status duringDIFS, the corresponding STA resumes the rest of the countdown. Thus, theAP transmits the unsolicited NDP frame/STF in the specific slot, therebypreventing channel accesses by arbitrary STAs.

Meanwhile, a PS-Poll interval is not configured for each STA indicatedby a TIM element, a total PS-Poll interval is configured only, and aPS-Poll operation can be performed on the basis of contention. This isdescribed in detail with reference to FIG. 40 as follows.

FIG. 40 is a diagram to describe a channel access operation of STAaccording to one embodiment of the present invention.

Referring to FIG. 40, only STAs (STA 1, STA 2, STA 3), which havechecked that data destined to the corresponding STAs were saved in an APthrough a TIM element, are able to contentionally perform PS-Polloperation in total PS-Poll interval. In doing so, the AP may be able toannounce information on a length and location of the total PS-Pollinterval to the STAs through TIM information. Yet, as mentioned in theforegoing description, the STA may be able to check the length andlocation of the total PS-Poll interval through a duration field of abeacon.

In the example shown in FIG. 40, the STA 1 selects a smallest backoffcount value, the STA 3 selects a greatest backoff count value, and theSTA 2 selects a backoff count value between the smallest and thegreatest. After the STA 1 has received the beacon, if the STA 1 confirmsthat a medium is in idle status during AIFS, the STA 1 counts down abackoff slot and then transmits a PS-Poll frame to the AP. Thereafter,each of the STA 2 and the STA 3 transmits a PS-Poll frame to the AP inthe same manner of the STA 1.

FIG. 41 is a diagram for one example of a channel access methodaccording to one embodiment of the present invention.

Referring to FIG. 41, an AP configures a channel access interval (e.g.,a PS-Poll interval) of each STA having a downlink data designatedthereto [S411].

The AP transmits a TIM element containing an information indicating apresence or non-presence of the downlink data, which is to betransmitted per STA in accordance with a beacon period, to STAs througha beacon frame [S413]. In this case, the TIM element can additionallyinclude a location information of a channel access interval for each ofthe STAs having the downlink data designated thereto by the TIM element,a start timing point information of total channel access interval, alength information of the channel access interval, an order informationof the channel access interval and the like.

The STA receives the beacon frame from the AP by switching to an awakestate in accordance with a beacon transmission period and then checks apresence or non-presence of the downlink data designated to thecorresponding STA and a channel access interval designated (assigned) tothe corresponding STA through the TIM element within the received beaconframe [S415].

Subsequently, the STA transmits a channel access frame (e.g., a PS-Pollframe) to the AP in the channel access interval designated to thecorresponding STA [S417]. The STA transmits the channel access frame tothe AP by maintaining the awake state only in the channel accessinterval designated to the corresponding STA within the total channelaccess interval. In this case, an NDP PS-Poll frame format can be usedfor the channel access frame.

In response to the channel access frame received from the STA, the APtransmits an ACK frame to the STA [S419]. In particular, the ACK framecan be transmitted in the PS-Poll interval of each of the STAs or may betransmitted to all STAs after the total channel access interval. In thiscase, a format of an NDP ACK frame or a group ACK frame (e.g., a PPGAframe) can be used for the ACK frame. Thereafter, the AP transmits thestored data to each of the STAs.

In the following description, if a channel access interval (e.g., aPS-Poll interval) of each STA is configured. A method of informing anSTA of the configured channel access interval is explained.

In case that channel access interval (e.g., a PS-Poll interval) of anSTA is configured, FIG. 42 shows one example of a method of informingthe STA of the configured channel access interval. FIG. 42 (a) showsthat 4 channel access intervals (e.g., PS-Poll intervals) for STA and atotal PS-Poll interval are configured. FIG. 42 (b) shows a method ofannouncing such information.

In particular, referring to FIG. 42 (b), an AP is able to transmit abeacon frame in a manner that a PS-Poll interval information for eachSTA and a total PS-Poll interval information are contained in the beaconframe, by which the present invention may be non-limited. Moreover,unlike the drawing, a PS-Poll interval information for each STA and atotal PS-Poll interval information can be transmitted in a manner ofbeing contained in a TIM. The AP can announce that a configured PS-Pollinterval is provided for which STA through a partial virtual bitmapcontained in a TIM element. For instance, through the partial virtualbitmap shown in FIG. 42 (b), the AP can indicate that PS-Poll intervalsfor 4 STAs shown in FIG. 42 (a) are provided for STA 1, STA 2, STA 4 andSTA 5, respectively. in other words, the STA 1, STA 2, STA 4 and STA 5are paged by the partial virtual bitmap and locations of the PS-Pollintervals for the respective STAs can be determined in the STA pagedorder in the partial virtual bitmap. In the above example, if the totalPS-Poll interval information is not transmitted, the total PS-Pollinterval can be determined as (PS-Poll Interval for STA)*(Number of STAsPaged in Partial Virtual Bitmap). This is applicable to a case that aPS-Poll interval for each STA is provided for a single STA but is notapplicable to a case that a PS-Poll interval is provided for two or moreSTAs.

As mentioned in the above description, when channel access intervals forSTAs are configured, a method of informing a user equipment of thechannel access interval configuration is useful to a case that STAspaged in TIM need to perform channel accesses only in their own channelaccess intervals, respectively. For instance, such method is appropriatefor user equipments that should minimize power consumptions like asensor type STA. Yet, if the STAs paged in the TIM include an offloadingSTA as well as the sensor type STA, they need to be identifiablyindicated.

To this end, various kinds of methods are described as follows. In thefollowing description, a channel access method according to a relatedart (i.e., an STA paged in TIM performs a contention and then transmitsa PS-Poll frame) shall be named a 1^(st) type. And, a new channel accessmethod (i.e., a PS-Poll is transmitted in a PS-Poll interval configuredfor each STA) shall be named a 2^(nd) type.

For another instance, when intervals for enabling STAs to access like aPS-Poll interval are assigned to STAs, an information (i.e., anindicator) indicating whether a channel access interval assigned to eachof the STAs is provided for a sensor type STA or a offloading STA can beannounced to the STAs in a manner of being contained in the assignedinterval. If a sensor type is indicated by the corresponding indicator,the STAs use the new channel access method. If an offloading type isindicated, the STAs can perform PS-Poll transmission using an existingcontention-based method.

According to a 1^(St) case, a sensor type STA and offloading STAs areincluded in a partial virtual bitmap of TIM. In this case, an AP canannounce that an STA should perform a channel access through a method ofa prescribed type through a 1^(st) bitmap indicating PS-Polltransmission types of STAs individually. In particular, through the1^(St) bitmap, the AP can indicate that each STA indicating a presenceof a buffered traffic in the TIM corresponds to a 1^(st) type or a2^(nd) type. So to speak, the AP can transmit the 1^(st) bitmap(individual PS-Poll mode bitmap) for the user equipments, which arepaged with reference to the partial virtual bitmap of the TIM, to STA.In the 1^(st) bitmap, bit 0 may indicate the 1^(st) type and bit 1 mayindicate the 2^(nd) type. And, the 1^(st) bitmap can be transmitted in amanner of being contained in a beacon or a TIM IE of the beacon.

The 1^(st) bit mentioned in the above description is shown in FIG. 43.Referring to FIG. 43, a 1^(st) bitmap (individual PS-Poll mode bitmap)is transmitted through a beacon frame. An STA 1 checks a presence ornon-presence of data buffered for the STA 1 through a partial virtualbitmap. The STA 1 checks the 1^(St) bitmap and is then able to transmita PS-Poll frame in accordance with a bit value set to 1, i.e., a 2^(nd)type. In FIG. 43, an STA 4 transmits a PS-Poll frame in accordance witha 1^(st) type.

According to a 2^(nd) case, a partial virtual bitmap for at least onegroup/page exists in a beacon or TIM IE. And, STAs having the sameattribute (e.g., sensor type STAs) are included per group/page.

In this case, an information related to a PS-Poll transmission type canbe indicated per group through a 2^(nd) bitmap (group PS-Poll modebitmap). In particular, the 2^(nd) bitmap can indicate that each groupindicated as having a buffered traffic in the partial virtual bitmapcorresponds to a 1^(st) type or a 2^(nd) type. In the 2^(nd) bitmap, abit value set to 0 can indicate the 1^(st) type, while a bit value setto 1 can indicate the 2^(nd) type.

One example of the 2^(nd) bitmap mentioned in the above description isshown in FIG. 44. Referring to FIG. 44, if an STA receives a beacon, theSTA is aware that a group to which the STA belongs is paged [page 1] andthen checks the 2^(nd) bitmap (group PS-Poll mode bitmap). As a bitvalue corresponding to the page 1 in the 2^(nd) bitmap is set to 1, theSTA transmits a PS-Poll frame in accordance with the 2^(nd) type.

The 2^(nd) bitmap mentioned in the above description can be transmittedin a manner of being contained in a beacon frame aside from a TIM for agroup.

FIG. 45 shows another example of the 2^(nd) case. Referring to FIG. 45(a), a PS-Poll mode indicator for a page is contained in a TIM. A group(page 2) having an indicator value set to 0 can transmit a PS-Poll inaccordance with a 1^(st) type and a group (page 2) having an indicatorvalue set to 1 can transmit a PS-Poll in accordance with a 2^(nd) type.FIG. 45 (b) shows one example that PS Poll mode information on a page iscontained in a TIM. Referring to FIG. 45 (b), a bitmap control field anda partial virtual bitmap field are included in the TIM. And, the partialvirtual bitmap consists of at least one or more blocks. The bitmapcontrol field includes a page index (group information) and a PS-Polltransmission type information (i.e., PS-Poll mode indicator) for thecorresponding page. If a PS-Poll type field is set to 1, it operates asa 1^(st) type. If a PS-Poll type field is set to 0, it operates as a2^(nd) type.

According to a 3^(rd) case, a TIM for at least one group is contained ina beacon. A specific group consists of user equipments having the sameattribute. A different group consists of user equipments havingdifferent attributes.

In this case, information related to a PS-Poll transmission type mayinclude a 1^(st) bitmap (individual PS-Poll mode bitmap) indicatingPS-Poll transmission types of STAs included in a group per STA, a 2^(nd)bitmap (group PS-Poll mode bitmap) indicating PS-Poll transmission typesof STAs included in groups per group, and a 3^(rd) bitmap (individualPS-Poll mode bitmap) indicating that a paged group is related to the1^(st) bitmap or the 2^(nd) bitmap. For instance, if the number ofgroups paged in TIM IE is 2, the 3^(rd) bitmap is configured with 2bits, a bit value ‘0’ can indicate that a PS-Poll transmission type isdetermined by the 2^(nd) bitmap, and a bit value ‘1’ can indicate that aPS-Poll transmission type is determined by the 1^(st) bitmap. Inparticular, the bit value ‘1’ indicates a group including an STA havinga different attribute. Based on a value set to 0 in the 3^(rd) bitmap, asize of the 2^(nd) bitmap can be determined. For instance, if a singlebit is set o 0 only in the 3^(rd) bitmap having a 2-bit size, the 2^(nd)bitmap is configured with 1 bit. And, 1^(st) bitmaps amounting to thenumber of bits set to 1 in the 3^(rd) bitmap are included.

FIG. 46 shows one example of PS-Poll IE including the 1^(st) to 3^(rd)bitmaps mentioned in the above description. In FIG. 46, assume a casethat there are 4 groups paged by TIM. After an STA has checked the3^(rd) bitmap, if a bit value is 0, the STA can determine a PS-Polltransmission type through the corresponding 2^(nd) bitmap. If a bitvalue is 1, the STA can determine a PS-Poll transmission type throughthe corresponding 1^(st) bitmap.

Although the above descriptions are made basically on the assumption ofa 1^(st) case that two PS-Poll transmission types (i.e., type and 2^(nd)type) exist, there may exist a PS-Poll transmission type of a 3^(rd)type. In this case, the 3^(rd) type is a PS-Poll transmission typedifferent from each of the 1^(st) type and the 2^(nd) type. And, the3^(rd) type may include a PS-Poll transmission type that can be derivedfrom one of the aforementioned various methods or the description in thepresent specification. In this case, the 2^(nd) and 3^(rd) types exceptthe 1^(st) type in an existing system are merged into an enhancedPS-Poll type. And, in the foregoing descriptions, a case of indicatingthe enhanced PS-Pol type can apply by replacing a case of indicating the2^(nd) bitmap among the 1^(st) to 3^(rd) bitmaps. This is described withreference to FIG. 47 and FIG. 48 as follows.

FIG. 47 shows a case that the 1^(st) map mentioned in the foregoingdescription supports an enhanced PS-Poll type. In particular, if a valueof a 1^(st) bitmap (individual PS-Poll mode bitmap) is 1, it indicatesan enhanced PS-Poll type. If the value is 0, it indicates a 1^(st) type.If the enhanced PS-Poll type is indicated, it is able to indicate a2^(nd) type (a case of a bit value set to 0) or a 3^(rd) type (a case ofa bit value set to 1) in an enhanced PS-Poll type bitmap.

FIG. 48 shows a case that the 2^(nd) map mentioned in the foregoingdescription supports an enhanced PS-Poll type. Referring to FIG. 48 (a),if a value of a 2^(nd) bitmap (group PS-Poll mode bitmap) is 1, itindicates an enhanced PS-Poll type. And, an enhanced PS-Poll type bitmapcan indicate a 2^(nd) type (a case of a bit value set to 0) or a 3^(rd)type (a case of a bit value set to 1). FIG. 48 (b) shows one examplethat PS Poll mode information on a page is included in a TIM.

FIG. 49 shows a case that the aforementioned PS-Poll IE supports anenhanced PS-Poll type. In particular, if a bit value is 1 in a 2^(nd)bitmap, one of a 2^(nd) type (a case of a bit value set to 0) and a3^(rd) type (a case of a bit value set to 1) can be indicated through agroup E-PMB bitmap. If a bit value of 1^(st) bitmaps (Individual PMB 1,Individual PMB 2) is 1, one of a 2^(nd) type (a case of a bit value setto 0) or a 3^(rd) type (a case of a bit value set to 1) can be indicatedin individual E-PMB bitmaps (Individual E-PMB 1, Individual E-PMB 2).

Meanwhile, a channel access can be granted only to user equipmentsbelonging to a specific group in a specific channel access interval.When STAs belonging to the corresponding group transmit PS-Poll in thechannel access interval, an AP can determine whether to allow a 1^(st)type or a 2^(nd) type to be used. In particular, when the AP assigns achannel access interval, the AP can information a user equipment of aPS-Poll type information. FIG. 50 shows one example that a PS-Poll typeinformation to be used for RAW is included when a channel accessinterval (e.g., restrict access window (RAW)) is assigned. In FIG. 50, apoll type field indicates a PS Poll operation type in a correspondingRAW, ‘0’ indicates a 1^(st) type, ‘1’ indicates a 2^(nd) type, andPS-Poll is transmitted in accordance with a corresponding type. Ofcourse, a poll type field value set to 0 may indicate the 1^(st) type.And, a poll type field value set to 1 may indicate a PS-Poll type.

Additional Channel Access Interval Assignment

According to the above-described embodiment, total PS-Poll interval canbe understood as RAW for transmitting PS-Poll, and PS-Poll interval foreach STA can be understood as slot in RAW. For clarity of thedescription, RAW for transmitting PS-Poll shall be named PS-Polldedicated RAW.

An STA can check whether data buffered for the corresponding STA existsthrough a TIM contained in a beacon frame. If the buffered data exists,the STA can confirm a PS-Poll dedicated RAW assigned to thecorresponding STA through a RAW information contained in the beaconframe. At least one of the number of RAWs to be assigned, a start timingpoint information of an assigned RAW, a slot (i.e., each PS-Pollinterval) duration information of a slot per RAW, and an orderinformation of each slot can be included in the RAW informationtransmitted through the beacon frame.

For instance, FIG. 51 is a diagram to describe an RAW assigned for abeacon interval. FIG. 51( a) shows one example that 2 RAWs (RAW 1, RAW2) are assigned for a beacon interval. The RAW 1 is the RAW assigned totransmit a PS-Poll frame or a trigger frame for example. And, the RAW 2is the RAW assigned to transmit data for example. A slot duration andboundary may be configured different per RAW. For instance, referring toFIG. 51, the slot duration of the RAW 1 is Ts1, whereas the slotduration of the RAW 2 can be set to Ts2 longer than Ts1.

FIG. 51( b) is a diagram for one example to describe a slot duration anda slot boundary. An EDCA based STA can perform a channel access for aslot duration. In doing so, the channel access in not performed across aslot boundary. In particular, the channel access is not performed acrossa plurality of slot durations. And, the STA may not wait for a probedelay when waking up on the slot boundary.

A paged STA checks an RAW through a beacon frame and is then able tomake a request for an AP to transmit data by transmitting a PS-Poll in aPS-Poll interval assigned to the paged STA. Yet, every STA may not beable to transmit a PS-Poll in its own PS-Poll interval. For instance, ifinterference caused by OBSS transmission is generated or an STA misses aTIM, the STA may not be able to transmit a PS-Poll in a PS-Poll intervalassigned to the corresponding STA. Having failed in transmitting thePS-Poll in its PS-Poll interval, the STA can perform a contention-basedPS-Poll. In doing so, if the contention-based PS-Poll is performed, itmay affect a PS-Poll performed by a different STA. This is described indetail with reference to examples shown in FIG. 52 and FIG. 53.

FIG. 52 and FIG. 53 are diagrams for one example of a case that aprescribed STA is unable to perform PS-Poll in its own PS-Poll interval.Referring to FIG. 52 and FIG. 53, a partial virtual bitmap field in aTIM of a beacon frame pages STA 1, STA 2, STA 4 and STA 5 for example.Hence, a PS-Poll dedicated RAW may be able to include at least 4 slots(i.e., 4 PS-Poll intervals) to match the number of the paged STAs. Eachof the STAs 1, 2, 4 and 5 can perform a STAs 1, 2, 4 and 5 in a PS-Pollinterval assigned to the corresponding STA. in FIG. 52 and FIG. 53, thePS-Poll intervals are assigned in order of the STAs 1, 2, 4 and 5 withinthe RAW for example.

If at least one STA is unable to transmit a PS-Poll in its own PS-Pollinterval due to the OBSS interference or a medium preoccupation by adifferent STA, the STA failing in the PS-Poll frame transmission canattempt a transmission of a PS-Poll frame through contention during aremaining PS-Poll dedicated RAW.

For instance, like the examples shown in FIG. 52 and FIG. 53, if the STA2 fails in transmitting a PS-Poll frame in a PS-Poll interval assignedto the STA 2, the STA 2 may be able to attempt a transmission of PS-Pollthrough contention with the STA 4 and the STA 5 during a remainingPS-Poll dedicated RAW.

In doing so, like the example shown in FIG. 52, if the STA 2 transmits aPS-Poll frame in a PS-Poll interval assigned for the STA 4, since theSTA 4 needs to maintain an awake state in a PS-Poll interval assignedfor the STA 5 in order to transmit a PS-Poll, the STA 4 may consume anadditional power. Moreover, since the STA 4 should attempt atransmission of a PS-Poll through a contention with the STA 5, at leastone of the STA 4 and the STA 5 will be unable to transmit a PS-Pollframe during a PS-Poll dedicated RAW.

Like the example shown in FIG. 53, assuming that the STA 5 is a hiddennode of the STA 2, if the STA 2 transmits a PS-Poll in a PS-Pollinterval assigned for the STA 5, a PS-Poll frame transmitted by the STA2 and a PS-Poll frame transmitted by the STA 5 collide with each otherso that both of the STA 2 and the STA 5 may fail in the transmission ofthe PS-Poll frames.

As a method of overcoming the inefficient contention mentioned in theabove description, an AP can additionally assign an additional RAW foran STA failing in a transmission of a PS-Poll frame behind a PS-Polldedicated RAW. In the following description, the additional RAW isexplained in detail.

FIG. 54 is a diagram for one example of a channel access method using anadditional RAW according to one embodiment of the present invention.

Referring to FIG. 54, an AP is able to configure PS-Poll dedicated RAWsfor channel accesses of downlink data designated STAs and an additionalRAW for an STA failing in a channel access [S511]. The AP is able todefine the PS-Poll dedicated RAW in order to assign a single slot toeach paged STA. On the other hand, when the AP defines the additionalRAW, the AP may define the additional RAW as a single slot. Inparticular, unlike the PS-Poll dedicated RAW, it is unnecessary for theadditional RAW to be defined as slots matching the number of the pagedSTAs. In case that the additional RAW is defined as a single slot, alength of the additional RAW may be set equal to that of the singleslot.

The AP can transmit a beacon frame containing a TIM element, whichcontains an information indicating a presence or non-presence ofdownlink data to be transmitted per STA in accordance with a beaconperiod, and an RAW information to the STA [S513].

In this case, the RAW information may include a PS-Poll dedicated RAWinformation and an additional RAW information. The PS-Poll dedicated RAWinformation can include at least one of a location information of aPS-Poll interval for each STA having downlink data designated thereto, astart timing point information of a PS-Poll dedicated RAW, a lengthinformation of the PS-Poll interval, and an order information of thePS-Poll interval. And, the additional RAW information can include atleast one of a start timing point information of an additional RAW, alength information of the additional RAW, and the number information ofa slot defined in the additional RAW.

The STA switches to an awake state to match a beacon transmission periodand is then able to receive a beacon frame from the AP. Having receivedthe beacon frame, the STA can check a presence or non-presence ofdownlink data designated to the corresponding STA through a TIM elementin the received beacon frame and is able to confirm a PS-Poll interval(i.e., slot) assigned to the corresponding STA within the PS-Polldedicated RAW and an additional RAW [S515].

The paged STA can transmit a PS-Poll frame in the slot (i.e., PS-Pollinterval) assigned to the paged STA within the PS-Poll dedicated RAW. Ifthe STA is unable to transmit the PS-Poll frame successfully in thePS-Poll interval assigned to the corresponding STA [S517], the STA cantransmit the PS-Poll frame during the additional RAW [S519]. If the STAfails to receive an acknowledgement (ACK) frame in the PS-Poll intervalin response to the PS-Poll frame (e.g., if not receiving the ACK framedespite a lapse of SIFS after the transmission of the PS-Poll frame) orthe STA is unable to transmit the PS-Poll frame in a PS-Polltransmission interval assigned to the corresponding STA due to the PBSSinterference, the STA can determine that the PS-Poll frame is notsuccessfully transmitted.

During the additional RAW, the STA can attempt a transmission of aPS-Poll frame based on contention (i.e., based on EDCA). For instance,if there are a plurality of STAs failing to transmit PS-Poll frame, aplurality of the STAs can attempt the transmission of the PS-Poll frameby contending with each other.

The STA failing to transmit the PS-Poll frame successfully through thePS-Poll dedicated RAW is able to reduce unnecessary power consumption bymaintaining a sleep state until the additional RAW starts.

Having transmitted the PS-Poll during the additional RAW, the STA canreceive an ACK frame in response to the transmitted PS-Poll [S521].

FIG. 55 and FIG. 56 are diagrams for one example when an additional RAWis applied. For clarity of the description, as mentioned in theforegoing descriptions with reference to FIG. 52 and FIG. 53, assumethat paged STAs include STA 1, STA 2, STA 4 and STA 5. And, assume thatPS-Poll intervals are assigned in order of STAs 1, 2, 4 and 5 within aPS-Poll dedicated RAW.

For instance, if the STA 2 fails in transmitting a PS-Poll in its ownPS-Poll interval due to OBSS interference, the STA 2 can attempt atransmission of a PS-Poll frame through an additional RAW assignedbehind a PS-Poll dedicated RAW. In this case, like the example shown inFIG. 55, the additional RAW may be situated right next to the PS-Polldedicated RAW. Alternatively, like the example shown in FIG. 56, theadditional RAW may be situated after a PS-Poll dedicated RAW fortransmitting a PS-Poll frame and a downlink (DL) data RAW fortransmitting a buffered data frame have ended.

Like the above-described example, if the STA fails to successfullytransmit the PS-Poll frame during the PS-Poll dedicated RAW, the STA canattempt the transmission of the PS-Poll frame again using the additionalRAW. As mentioned in the foregoing description, only the paged STAfailing in successfully transmitting the PS-Poll in its own slot (e.g.,its own PS-Poll interval) within the PS-Poll dedicated RAW can utilizethe additional RAW as a RAW for a channel access.

Yet, if the additional RAW does not end despite that every paged STAsuccessful transmits the PS-Poll frames during the PS-Poll dedicated RAWor the STA failing in transmitting the PS-Poll during the PS-Polldedicated RAW transmits the PS-Poll through the additional RAW, theadditional RAW results in wasting the limited radio resources instead.

Hence, an AP can enable at least one of a paged STA failing insuccessfully transmitting a PS-Poll, a paged STA succeeding intransmitting a PS-Poll, and an unpaged STA to transmit an uplink framethrough an additional RAW. To this end, if the AP determines that everypaged STA has successfully transmitted the PS-Poll frames, if the APdetects that a channel is in an idle status over a specific period sincea start of the additional RAW, or as soon as the additional RAW starts,the AP is able to transmit a UTA (UL transmission allowance) frame toinform STAs that a transmission of an uplink frame is allowed. In doingso, the AP can transmit the UTA frame by broadcast or may transmit theUTA frame by unicast only to prescribed STAs to allow for the uplinkframe transmission.

Having received the UTA frame, the STA can transmit an uplink frame tothe AP based on contention (i.e., EDCA) during the additional RAW. Thisis described in detail with reference to FIG. 57 and FIG. 58 as follows.

FIG. 57 and FIG. 58 are diagrams for one example of transmitting anuplink frame to an AP from an STA for an additional RAW. For clarity ofthe following description, assume that STAs paged through a partialvirtual map include STA 1, STA 2 and STA 4.

An AP is able to transmit a UTA frame to announce allowance for anuplink transmission of an STA during an additional RAW. Like the exampleshown in FIG. 57, the UTA frame can be transmitted to a paged STA onlyby unicast (or multicast) Like the example shown in FIG. 58, the UTAframe can be transmitted to both a paged STA and an unpaged STA bybroadcast as well as to the paged STA.

Having received the UTA frame, the STAs recognize that the additionalRAW in idle status and are then able to attempt transmissions of uplinkframes through contention during the additional RAW. During theadditional RAW, the AP may be able to receive the UL frame from at leastone of the paged STA and the unpaged STA.

The UTA frame may have a MAC control frame format or include a frame inNDP (null data packet) format. For instance, FIG. 59 is a diagram forone example of an NDP UTA frame format. Referring to FIG. 59, the NDPUTA frame may include STF, LTF and SIG field. The SIG field may includean NDP frame type information indicating that a corresponding NDP frameis a UTA frame. An STA is able to confirm that the NDP frame is the UTAframe by checking the NDP frame type information of the SIG field. Inorder for the NDP frame type information to indicate the UTA frame, itis able to use a reserved bit of an MCS field that is a subfield of theSIG field. Moreover, existing frames (e.g., CTS MAC control frame,NDP-CTS frame, CF-END frame, etc.) can be transmitted by including theUTA frame function. Instead, the existing frame is transmitted bycontaining an information (e.g., indication bit or duration set to 0)indicating that channel accesses of other STAs are allowed after acorresponding timing point.

The SIG field may further include a BSSID information or a partial BSSIDinformation. The BSSID information or the partial BSSID information mayindicate a BSSID to which an NDP UTA frame transmitting AP belongs.Having received the NDP UTA frame, an STA can attempt a transmission ofan uplink frame during an additional RAW only if belonging to the sameBSSID of the STA.

Although the ATP is able to attempt a reception of the uplink frame fromthe STA during the additional RAW, the AP may be able to transmit adownlink data frame to the STA during the additional RAW. This isdescribed in detail with reference to FIG. 60 as follows.

FIG. 60 is a diagram for one example of transmitting a downlink dataframe to an STA from an AP for an additional RAW. For clarity of thefollowing description, assume that STAs paged through a partial virtualbitmap include STA 1, STA 2 and STA 4.

If detecting that a channel is in an idle status during a specificperiod since an additional RAW starts, an AP can transmit a downlinkdata frame to an STA. Although FIG. 60 shows one example that thedownlink data frame is transmitted to the unpaged STA 5, if there isdata to be transmitted to the paged STA, the downlink data frame can betransmitted during the additional RAW.

Despite that the downlink data frame was transmitted to the STA, if thechannel is still in the idle status, the AP can transmit a UTA frame,which indicate that an uplink frame is transmittable during theadditional RAW, for the first time by transmitting a UTA frame to theSTA. In doing so, as mentioned in the foregoing descriptions withreference to FIG. 57 and FIG. 58, the UTA frame may be transmitted to apaged STA or may be transmitted to an unpaged STA as well as to thepaged STA.

As mentioned in the foregoing description, an STA failing intransmitting a PS-Poll frame in its own PS-Poll interval can attempt atransmission of a PS-Poll frame during an additional RAW. In doing so,an AP transmits a UTA frame during the additional RAW so that at leastone of a paged STA succeeding in transmitting a PS-Poll frame and anunpaged STA can use the additional RAW. Yet, in order for an STA failingin transmitting a PS-Poll frame to transmit an uplink frame (i.e.,PS-Poll) through the additional RAW earlier than other STAs (e.g., apaged STA succeeding in transmitting a PS-Poll frame and an unpagedSTA), it is able to give a channel access priority to the paged STAfailing in the transmission of the PS-Poll frame. To this end, EDCAparameter can be newly defined for PS-Poll traffic.

Table 2 to 5 show examples of EDCA parameters for PS-Poll traffic.

TABLE 2 Access Category (AC) CWmin CWmax AIFSN AC_BK aCWmin aCWmax 7AC_BE aCWmin aCWmax 3 AC_VI (aCWmin + 1)/2 − 1 aCWmin 2 AC_VO (aCWmin +1)/4 − 1 (aCWmin + 1)/2 − 1 2 AC_PS-Poll (aCWmin + 1)/8 − 1 (aCWmin +1)/4 − 1 1

Referring to Table 2, for a PS-Poll transmission, a new access categoryAC_PS-Poll can be defined. By setting EDCA parameter (e.g., CWmin,CWmax, AIFSN (Arbitration Inter-Frame Spacing Number)) of an accesscategory for PS-Poll transmission to a value lower than an accesscategory of traffic (e.g., Background (BK), Best Effort (BE), Video(VI), Voice (VO)) of another kind, a paged STA performing PS-Poll in aPS-Poll dedicated RAW and an additional RAW can perform a channel accessmore quickly.

TABLE 3 Access Category (AC) CWmin CWmax AIFSN AC_BK aCWmin aCWmax 7AC_BE aCWmin aCWmax 3 AC_VI (aCWmin + 1)/2 − 1 aCWmin 2 AC_VO (aCWmin +1)/4 − 1 (aCWmin + 1)/2 − 1 2 AC_PS-Poll (aCWmin + 1)/4 − 1 (aCWmin +1)/2 − 1 1

Referring to Table 3, CWmin and CWmax of an access category for PS-Polltransmission are set equal to an access category of audio traffic. And,AIFSN can be set lower than the access category of the audio traffic.

TABLE 4 Access Category (AC) CWmin CWmax AIFSN AC_BK aCWmin aCWmax 7AC_BE aCWmin aCWmax 3 AC_VI (aCWmin + 1)/2 − 1 aCWmin 2 AC_VO (aCWmin +1)/4 − 1 (aCWmin + 1)/2 − 1 2 AC_PS-Poll (aCWmin + 1)/8 − 1 (aCWmin +1)/4 − 1 2

Referring to table 4, CWmin and CWmax of an access category for PS-Polltransmission are set different from an access category of audio traffic.And, AIFSN can be set equal to the access category of the audio traffic.

TABLE 5 Access Category (AC) CWmin CWmax AIFSN AC_BK aCWmin aCWmax 7AC_BE aCWmin aCWmax 3 AC_VI (aCWmin + 1)/2 − 1 aCWmin 2 AC_VO/AC_PS-(aCWmin + 1)/4 − 1 (aCWmin + 1)/2 − 1 2 Poll

Referring to Table 5, EDCA parameter of an access category for PS-Polltransmission may be set equal to an access category of audio traffic.Hence, PS-Poll will have a priority equivalent to that of audio traffic.

Referring to the descriptions with reference to Table 2 to 5, PS-Pollhas a priority equal to or higher than that of audio traffic. Thedescriptions with reference to Table 2 to 5 are exemplarily made forclarity of the description, by which the present invention may benon-limited. And, it is a matter of course that PS-Poll can be set tohave a priority lower than that of audio traffic.

The EDCA parameter for the PS-Poll traffic mentioned in the descriptionswith reference to Tables 2 to 5 are available for a paged user equipmentto transmit PS-Poll during a PS-Poll dedicated RAW as well as during anadditional RAW. Hence, an STA can transmit PS-Poll more quickly duringthe PS-Poll dedicated RAW.

As an additional method of giving a priority to a PS-Poll frametransmission, it is able to consider a method for a paged user equipmentto sense a busy (occupied) status of a medium before a PS-Poll frametransmission not during AIFS but during DIFS or PIFS. In this case,since the paged user equipment can transmit a PS-Poll frame after lapseof DIFS or PIFS shorter than AIFS, it may be possible to transmit aPS-Poll frame more quickly.

FIG. 61 is a block diagram for a configuration of a wireless deviceaccording to one embodiment of the present invention.

An AP 10 may include a processor 11, a memory 12 and a transceiver 13.An STA 20 may include a processor 21, a memory 22 and a transceiver 23.The transceiver 13/23 can transmit and receive radio signals and is ableto implement a physical layer according to IEEE 802 system for example.The processor 11/21 is connected to the transceiver 13/23 and is able toimplement a physical layer and/or a MAC layer according to IEEE 802system. The processor 11/21 can be configured to perform operationsaccording to the various embodiments of the present invention mentionedin the foregoing description. And, modules for implementing operationsof the AP and STA according to the various embodiments of the presentinvention mentioned in the foregoing description are saved in the memory12/22 and can be executed by the processor 11/21. The memory 12/22 isincluded in the processor 11/21 or installed outside the processor 11/21and is then connected to the processor 11/21 via a means known to thepublic.

In the above-mentioned detailed configurations of the AP and STA device,the contents or items explained in the descriptions of the variousembodiments of the present invention may be independently applicable orat least two embodiments of the present invention may be simultaneouslyapplicable. And, redundant descriptions shall be omitted from thefollowing description for clarity.

Embodiments of the present invention can be implemented using variousmeans. For instance, embodiments of the present invention can beimplemented using hardware, firmware, software and/or any combinationsthereof.

In case of the implementation by hardware, methods according toembodiments of the present invention can be implemented by at least oneselected from the group consisting of ASICs (application specificintegrated circuits), DSPs (digital signal processors), DSPDs (digitalsignal processing devices), PLDs (programmable logic devices), FPGAs(field programmable gate arrays), processor, controller,microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, methods accordingto embodiments of the present invention can be implemented by modules,procedures, and/or functions for performing the above-explainedfunctions or operations. Software code is stored in a memory unit and isthen drivable by a processor. The memory unit is provided within oroutside the processor to exchange data with the processor through thevarious means known to the public.

As mentioned in the foregoing description, the detailed descriptions forthe preferred embodiments of the present invention are provided to beimplemented by those skilled in the art. While the present invention hasbeen described and illustrated herein with reference to the preferredembodiments thereof, it will be apparent to those skilled in the artthat various modifications and variations can be made therein withoutdeparting from the spirit and scope of the invention. Thus, it isintended that the present invention covers the modifications andvariations of this invention that come within the scope of the appendedclaims and their equivalents. Therefore, the present invention isnon-limited by the embodiments disclosed herein but intends to give abroadest scope matching the principles and new features disclosedherein.

INDUSTRIAL APPLICABILITY

Various embodiments according to the present invention are mainlydescribed with reference to the examples applying to IEEE 802.11 systemand can identically apply to various kinds of wireless access systems aswell as to IEEE 802.11 system.

1. A method of performing a channel access, which is performed by astation (STA) in a wireless communication system, comprising the stepsof: receiving a beacon frame containing a TIM (traffic indication map);and if the TIM indicates that a traffic for the STA is buffered,transmitting a PS-Poll (Power Save-Poll) frame, wherein the PS-Pollframe is transmitted during a first RAW (restricted access window)accessible for a paged STA, and wherein at least one of EDCA parametersapplied to transmitting the PS-Poll frame is equivalent to an EDCAparameter applied to transmitting an audio traffic. 2-20. (canceled) 21.The method of claim 1, wherein the EDCA parameter comprises at least oneselected from the group consisting of CWmin (minimum Contention Window),CWmax (maximum Contention Window) and AIFSN (Arbitration Inter-FrameSpacing Number).
 22. The method of claim 1, wherein the STA receives adata frame during a second RAW additionally assigned behind the firstRAW.
 23. The method of claim 1, wherein if the PS-Poll frame does not betransmitted during the first RAW, the STA transmits the PS-Poll frameduring a second RAW additionally assigned behind the first RAW.
 24. Themethod of claim 23, wherein if the STA fails to transmit the PS-Pollframe during the first RAW or is unable to receive an ACK(acknowledgement) frame in response to the PS-Poll despite transmittingthe PS-Poll frame during the first RAW, the STA determines that thePS-Poll frame was transmitted unsuccessfully during the first RAW. 25.The method of claim 23, wherein after transmitting the PS-Poll framesuccessfully, the STA receives a UTA (UL transmission allow) frame froman Access Point (AP) indicating that the channel access is allowedduring the second RAW.
 26. The method of claim 1, wherein if the TIMdoes not indicate that the traffic buffered for the STA or the STAsuccessfully transmits the PS-Poll frame during the first RAW, themethod further comprises: receiving, at the STA, a UTA (UL transmissionallowance) frame during a second RAW additionally assigned behind thefirst RAW, and attempting the channel access during the second RAW. 27.A method of performing a channel access, which is supported by an AP(access point) in a wireless communication system, comprising the stepsof: transmitting a beacon frame containing a TIM (traffic indicationmap); and receiving a PS-Poll (Power Save-Poll) frame from a paged STAreceiving an indication of a presence of a buffered traffic the TIM,wherein the PS-Poll frame is received during a first RAW (restrictedaccess window) accessible for a paged STA, and wherein at least one ofEDCA parameters applied to transmitting the PS-Poll frame is equivalentto an EDCA parameter applied to transmitting an audio traffic.
 28. Themethod of claim 27, wherein the EDCA parameter comprises at least oneselected from the group consisting of CWmin (minimum Contention Window),CWmax (maximum Contention Window) and AIFSN (Arbitration Inter-FrameSpacing Number).
 29. The method of claim 27, wherein the AP transmits adata frame during a second RAW additionally assigned behind the firstRAW.
 30. The method of claim 27, wherein if the PS-Poll frame does notbe received during the first RAW, the AP receives the PS-poll frameduring a second RAW additionally assigned behind the first RAW.
 31. Themethod of claim 27, wherein the AP transmits a UTA (UL transmissionallowance) frame indicating that the channel access of the STA isallowed during a second RAW additionally assigned behind the first RAW.32. The method of claim 31, wherein the UTA frame is transmitted while achannel is idle over a prescribed time during the second RAW.
 33. Themethod of claim 31, wherein the UTA frame is transmitted to the pagedSTA by unicast or multicast.
 34. The method of claim 31, wherein the UTAframe is transmitted by broadcast.
 35. In a station (STA) deviceconfigured to perform a channel access in a wireless communicationsystem, an apparatus comprising: a transceiver configured to transceivea radio signal; a processor configured to receive a beacon framecontaining a TIM (traffic indication map) from an AP (access point), theprocessor, if the TIM indicates that a traffic is buffered, configuredto transmit a PS-Poll (Power Save-Poll) frame to the AP, wherein thePS-Poll frame is transmitted a first RAW (restricted access window)accessible for a paged STA, and wherein at least one EDCA parameterapplied to transmitting the PS-Poll frame is equivalent to an EDCAparameter applied to transmitting an audio traffic.
 36. In an AP (accesspoint) device configured to support a channel access in a wirelesscommunication system, an apparatus comprising: a transceiver configuredto transceive a radio signal; a processor configured to transmit abeacon frame containing a TIM (traffic indication map), the processorconfigured to receive a PS-Poll frame from an STA receiving anindication of a buffered traffic by the TIM, wherein the PS-Poll frameis received during a first RAW (restricted access window) accessible fora paged STA, and wherein at least one EDCA parameter applied totransmitting the PS-Poll frame is equivalent to an EDCA parameterapplied to transmitting an audio traffic.